ffplay: Don't assume ic->pb is non-null

This fixes crashes on exit when plaing some RealRTSP streams
(among other), e.g.
rtsp://dl.lib.brown.edu:554/areserves/1093545294660883.mp3.
These crashes have been present since 3e68b3ba7b015cf21.

Signed-off-by: Ronald S. Bultje <rsbultje@gmail.com>
