avcodec/vc1dec: fix null pointer dereference

Fixes Ticket3860
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 30f680ee)
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/vc1dec.c

@@ -5484,7 +5484,7 @@ static int vc1_decode_sprites(VC1Context *v, GetBitContext* gb)
     if (ret < 0)
         return ret;
 
-    if (!s->current_picture.f->data[0]) {
+    if (!s->current_picture.f || !s->current_picture.f->data[0]) {
         av_log(avctx, AV_LOG_ERROR, "Got no sprites\n");
         return -1;
     }