avcodec/vc1dec: do not crash when flushing without an allocated frame

Fixes Ticket3837
Found-by: Piotr Bandurski <ami_stuff@o2.pl>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 6801eb0a09)

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/vc1dec.c

@@ -5514,7 +5514,7 @@ static void vc1_sprite_flush(AVCodecContext *avctx)
        Since we can't enforce it, clear to black the missing sprite. This is
        wrong but it looks better than doing nothing. */
 
-    if (f->data[0])
+    if (f && f->data[0])
         for (plane = 0; plane < (s->flags&CODEC_FLAG_GRAY ? 1 : 3); plane++)
             for (i = 0; i < v->sprite_height>>!!plane; i++)
                 memset(f->data[plane] + i * f->linesize[plane],