ffserver: Fix a null pointer dereference as a result of the FF_API_MAX_STREAMS cleanup.

Fixed another dereference in the RTSP code.

Removed a useless variable.

Changed an unnecessary looping assignment to a simple assignment suggested by
Maksym.

Added fixes and tweaks suggested by Maksym Veremeyenko [verem@m1stereo.tv] and
Clément B.
