avformat/rmdec: Fix potential crash on allocation failure

avformat/rmdec: Fix potential crash on allocation failure

The RealMedia demuxer uses the priv_data of its streams to store a
structure containing an AVPacket. These packets are unreferenced in the
read_close function, yet said function simply presumed that the
priv_data has been successfully allocated. This implies that it mustn't
be called when an allocation of priv_data fails; but this can happen
since commit 35bbc1955a58ba74552c50d9161084644f00bbd3 if one has a
stream with multiple substreams (also exported as AVStream) and if
allocating the priv_data for one of these substreams fails.

This has been fixed by making sure that read_close can handle the case
in which priv_data has not been successfully allocated.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>