avformat/rtspdec: add network init to listen mode

avformat/rtspdec: add network init to listen mode

As per the docs network initialization is required before ff_url_join().
Furthermore, because the ff_network_init() was skipped, this makes
one additional call to ff_network_close() if the stream exits without
errors.

The was forgotten in the origin commit of the listen mode:
a8ad6ffafe89e3a83f343f69249338e8245816f7

Reviewed-by: Martin Storsjö <martin@martin.st>
Signed-off-by: Andriy Gelman <andriy.gelman@gmail.com>