mov: Seek back if overreading an individual atom

Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Cc: libav-stable@libav.org
Signed-off-by: NMartin Storsjö <martin@martin.st>

libavformat/mov.c

@@ -2592,6 +2592,12 @@ static int mov_read_default(MOVContext *c, AVIOContext *pb, MOVAtom atom)
             left = a.size - avio_tell(pb) + start_pos;
             if (left > 0) /* skip garbage at atom end */
                 avio_skip(pb, left);
+            else if (left < 0) {
+                av_log(c->fc, AV_LOG_WARNING,
+                       "overread end of atom '%.4s' by %"PRId64" bytes\n",
+                       (char*)&a.type, -left);
+                avio_seek(pb, left, SEEK_CUR);
+            }
         }
 
         total_size += a.size;