Lib: fix undefined behavior in amqp_encode_bytes
Lib: fix undefined behavior in amqp_encode_bytes

Passing a NULL pointer to the src parameter of memcpy results in
undefined behavior even if the len parameter is 0. Fix is to check for
the length before attempting to memcpy.

Haster2004/rabbitmq-c,alanxz/rabbitmq-c,alanxz/rabbitmq-c,alanxz/rabbitmq-c,Haster2004/rabbitmq-c,Haster2004/rabbitmq-c