stunlib_DecodeMessage with NULL stream can crash

[clux]

This line will cause a crash when using stunlib with a NULL stream argument. i.e. stunlib_DecodeMessage(buff, bufLen, &msg, NULL, NULL). Pretty much everywhere in stunlib_DecodeMessage is guarding on stream != NULL) before calling printError, but in the case of an invalid crc, it does not.

I would suggest moving the NULL guard into printError to be less error prone and fix this use case.