avformat/rmdec: Do not pass mime type in rm_read_multi() to ff_rm_read_mdpr_codecdata()

Fixes: use after free()
Fixes: rmdec-crash-ffe85b4cab1597d1cfea6955705e53f1f5c8a362

Found-by: Paul Ch <paulcher@icloud.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit a7e032a277)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/rmdec.c

@@ -522,7 +522,7 @@ static int rm_read_multi(AVFormatContext *s, AVIOContext *pb,
 
         size2 = avio_rb32(pb);
         ret = ff_rm_read_mdpr_codecdata(s, s->pb, st2, st2->priv_data,
-                                        size2, mime);
+                                        size2, NULL);
         if (ret < 0)
             return ret;
     }