udp-turn: don't re-iterate incoming TURN control messages

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.