Fix a bug

Update value of variable local_rtp_port before checking if it is not less than zero

This bug makes the rtcp port and rtp port are not two successive integers which are sent to client as server ports.

But some video players on Android will not send udp hole punching messages if the rtcp port and rtp port are not two successive integers.So, if the video player is behind NAT, it cannot receive and rtp messages via udp.