vc1dec: Make sure last_picture is initialized in vc1_decode_skip_blocks

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

libavcodec/vc1dec.c

@@ -5143,6 +5143,9 @@ static void vc1_decode_skip_blocks(VC1Context *v)
 {
     MpegEncContext *s = &v->s;
 
+    if (!v->s.last_picture.f.data[0])
+        return;
+
     ff_er_add_slice(&s->er, 0, s->start_mb_y, s->mb_width - 1, s->end_mb_y - 1, ER_MB_END);
     s->first_slice_line = 1;
     for (s->mb_y = s->start_mb_y; s->mb_y < s->end_mb_y; s->mb_y++) {