[FIXED] Configuration Reload: possible panic if done during Shutdown

If a configuration reload is issued as the server is being shutdown,
we could get 2 different panics. One due to JetStream if an account
is JetStream enabled, and one due to the send to a go channel that
has been closed.

```
panic: send on closed channel [recovered]
        panic: send on closed channel

goroutine 440 [running]:
testing.tRunner.func1.2({0x1038d58e0, 0x1039e1270})
        /usr/local/go/src/testing/testing.go:1545 +0x274
testing.tRunner.func1()
        /usr/local/go/src/testing/testing.go:1548 +0x448
panic({0x1038d58e0?, 0x1039e1270?})
        /usr/local/go/src/runtime/panic.go:920 +0x26c
github.com/nats-io/nats-server/v2/server.(*Server).reloadAuthorization(0xc00024fb00)
        /Users/ik/dev/go/src/github.com/nats-io/nats-server/server/reload.go:1998 +0x788
github.com/nats-io/nats-server/v2/server.(*Server).applyOptions(0xc00024fb00, 0xc00021dc00, {0xc00038e4e0, 0x2, 0xc00021dc28?})
        /Users/ik/dev/go/src/github.com/nats-io/nats-server/server/reload.go:1746 +0x2b8
github.com/nats-io/nats-server/v2/server.(*Server).reloadOptions(0xc000293500?, 0xc000118a80, 0xc000293500)
        /Users/ik/dev/go/src/github.com/nats-io/nats-server/server/reload.go:1121 +0x178
github.com/nats-io/nats-server/v2/server.(*Server).ReloadOptions(0xc00024fb00, 0xc000293500)
        /Users/ik/dev/go/src/github.com/nats-io/nats-server/server/reload.go:1060 +0x368
github.com/nats-io/nats-server/v2/server.(*Server).Reload(0xc00024fb00)
        /Users/ik/dev/go/src/github.com/nats-io/nats-server/server/reload.go:995 +0x104
```

and

```
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x2 addr=0x0 pc=0x10077b224]

goroutine 8 [running]:
testing.tRunner.func1.2({0x101351640, 0x101b7d2a0})
	/usr/local/go/src/testing/testing.go:1545 +0x274
testing.tRunner.func1()
	/usr/local/go/src/testing/testing.go:1548 +0x448
panic({0x101351640?, 0x101b7d2a0?})
	/usr/local/go/src/runtime/panic.go:920 +0x26c
github.com/nats-io/nats-server/v2/server.(*Account).EnableJetStream(0xc00020fb80, 0xc000220240)
	/Users/ivan/dev/go/src/github.com/nats-io/nats-server/server/jetstream.go:1045 +0xa4
github.com/nats-io/nats-server/v2/server.(*Server).configJetStream(0xc000226d80, 0xc00020fb80)
	/Users/ivan/dev/go/src/github.com/nats-io/nats-server/server/jetstream.go:707 +0xdc
github.com/nats-io/nats-server/v2/server.(*Server).configAllJetStreamAccounts(0xc000226d80)
	/Users/ivan/dev/go/src/github.com/nats-io/nats-server/server/jetstream.go:768 +0x2b0
github.com/nats-io/nats-server/v2/server.(*Server).enableJetStreamAccounts(0xc000226d80?)
	/Users/ivan/dev/go/src/github.com/nats-io/nats-server/server/jetstream.go:637 +0x128
github.com/nats-io/nats-server/v2/server.(*Server).reloadAuthorization(0xc000226d80)
	/Users/ivan/dev/go/src/github.com/nats-io/nats-server/server/reload.go:2039 +0x93c
github.com/nats-io/nats-server/v2/server.(*Server).applyOptions(0xc000226d80, 0xc000171c50, {0xc000074600, 0x2, 0xc000171c78?})
	/Users/ivan/dev/go/src/github.com/nats-io/nats-server/server/reload.go:1746 +0x2b8
github.com/nats-io/nats-server/v2/server.(*Server).reloadOptions(0xc000276000?, 0xc0000a6000, 0xc000276000)
	/Users/ivan/dev/go/src/github.com/nats-io/nats-server/server/reload.go:1121 +0x178
github.com/nats-io/nats-server/v2/server.(*Server).ReloadOptions(0xc000226d80, 0xc000276000)
	/Users/ivan/dev/go/src/github.com/nats-io/nats-server/server/reload.go:1060 +0x368
github.com/nats-io/nats-server/v2/server.(*Server).Reload(0xc000226d80)
	/Users/ivan/dev/go/src/github.com/nats-io/nats-server/server/reload.go:995 +0x104
```

Signed-off-by: Ivan Kozlovic <ivan@synadia.com>

server/events.go

@@ -1598,14 +1598,16 @@ func (s *Server) shutdownEventing() {
 	s.mu.Lock()
 	clearTimer(&s.sys.sweeper)
 	clearTimer(&s.sys.stmr)
-	sys := s.sys
+	rc := s.sys.resetCh
+	s.sys.resetCh = nil
+	wg := &s.sys.wg
 	s.mu.Unlock()
 
 	// We will queue up a shutdown event and wait for the
 	// internal send loop to exit.
 	s.sendShutdownEvent()
-	sys.wg.Wait()
-	close(sys.resetCh)
+	wg.Wait()
+	close(rc)
 
 	s.mu.Lock()
 	defer s.mu.Unlock()

server/jetstream.go

@@ -1038,6 +1038,10 @@ func (a *Account) EnableJetStream(limits map[string]JetStreamAccountLimits) erro
 	}
 
 	s.mu.RLock()
+	if s.sys == nil {
+		s.mu.RUnlock()
+		return ErrServerNotRunning
+	}
 	sendq := s.sys.sendq
 	s.mu.RUnlock()
 

server/reload_test.go

@@ -6100,3 +6100,41 @@ func TestConfigReloadLeafNodeCompressionS2Auto(t *testing.T) {
 		t.Fatalf("Expected compression info to have stayed the same, was %q - %p, got %q - %p", cm, cw, ncm, ncw)
 	}
 }
+
+func TestConfigReloadNoPanicOnShutdown(t *testing.T) {
+	tmpl := `
+		port: -1
+		jetstream: true
+		accounts {
+			A {
+				users: [{user: A, password: pwd}]
+				%s
+			}
+			B {
+				users: [{user: B, password: pwd}]
+			}
+		}
+	`
+	for i := 0; i < 50; i++ {
+		conf := createConfFile(t, []byte(fmt.Sprintf(tmpl, _EMPTY_)))
+		s, _ := RunServerWithConfig(conf)
+		// Don't use a defer s.Shutdown() here since it would prevent the panic
+		// to be reported (but the test would still fail because of a runtime timeout).
+
+		err := os.WriteFile(conf, []byte(fmt.Sprintf(tmpl, "jetstream: true")), 0666)
+		require_NoError(t, err)
+
+		wg := sync.WaitGroup{}
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			time.Sleep(10 * time.Millisecond)
+			s.Shutdown()
+		}()
+
+		time.Sleep(8 * time.Millisecond)
+		err = s.Reload()
+		require_NoError(t, err)
+		wg.Wait()
+	}
+}