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 35bbc1955a 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>
(cherry picked from commit 5aafdb4e5f)

libavformat/rmdec.c

@@ -115,6 +115,9 @@ RMStream *ff_rm_alloc_rmstream (void)
 
 void ff_rm_free_rmstream (RMStream *rms)
 {
+    if (!rms)
+        return;
+
     av_packet_unref(&rms->pkt);
 }