udp-turn: don't re-iterate incoming TURN control messages
After being parsed, a TURN control message turns into a NiceInputMessage with zero length. Such message doesn't increment the iteration counter i and so is re-processed in the next iteration, which detects right away that message->length == 0 and continues to the next element in recv_messages. Thus, n_valid_messages variable serves no real purpose and to achieve the same result we can simply increment the iteration counter after each message.
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