avcodec/mpeg4videodec: also for empty partitioned slices

Fixes assertion failure
Fixes: id_acf3e47f864e1ee4c7b86c0653e0ff31e5bde56e.m4v

Found-by: Andreas Cadhalpun <andreas.cadhalpun@googlemail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 70f13abb4f)
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>

libavcodec/mpeg4videodec.c

@@ -881,7 +881,7 @@ int ff_mpeg4_decode_partitions(Mpeg4DecContext *ctx)
     const int part_a_end   = s->pict_type == AV_PICTURE_TYPE_I ? (ER_DC_END   | ER_MV_END)   : ER_MV_END;
 
     mb_num = mpeg4_decode_partition_a(ctx);
-    if (mb_num < 0) {
+    if (mb_num <= 0) {
         ff_er_add_slice(&s->er, s->resync_mb_x, s->resync_mb_y,
                         s->mb_x, s->mb_y, part_a_error);
         return -1;