avformat/rmdec: Fix memleaks upon read_header failure

avformat/rmdec: Fix memleaks upon read_header failure

For both the RealMedia as well as the IVR demuxer (which share the same
context) each AVStream's priv_data contains an AVPacket that might
contain data (even when reading the header) and therefore needs to be
unreferenced. Up until now, this has not always been done:

The RealMedia demuxer didn't do it when allocating a new stream's
priv_data failed although there might be other streams with packets to
unreference. (The reason for this was that until recently rm_read_close()
couldn't handle an AVStream without priv_data, so one had to choose
between a potential crash and a memleak.)

The IVR demuxer meanwhile never ever called read_close so that the data
already contained in packets leaks upon error.

This patch fixes both demuxers by adding the appropriate cleanup code.

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