Corrected -Wsign-compare warnings on ESP8266 under platformio.
/home/travis/.platformio/lib/Crypto_ID1168/RNG.cpp: In member function 'void RNGClass::rand(uint8_t*, size_t)':
/home/travis/.platformio/lib/Crypto_ID1168/RNG.cpp:574:27: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
if (len > (credits / 8))
^
/home/travis/.platformio/lib/Crypto_ID1168/RNG.cpp: In member function 'bool RNGClass::available(size_t) const':
/home/travis/.platformio/lib/Crypto_ID1168/RNG.cpp:665:35: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
return len <= (credits / 8);
^
/home/travis/.platformio/lib/Crypto_ID1168/RNG.cpp: In member function 'void RNGClass::stir(const uint8_t*, size_t, unsigned int)':
/home/travis/.platformio/lib/Crypto_ID1168/RNG.cpp:698:39: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
if ((RNG_MAX_CREDITS - credits) > credit)
^
