avformat/rmdec: Fix Packet memleak at close()

Fixes: asan_heap-oob_445b39_1741_d00eb645ab48eb2203b4a04a5b997103.ivr

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/rmdec.c

@@ -1399,5 +1399,6 @@ AVInputFormat ff_ivr_demuxer = {
     .read_probe     = ivr_probe,
     .read_header    = ivr_read_header,
     .read_packet    = ivr_read_packet,
+    .read_close     = rm_read_close,
     .extensions     = "ivr",
 };