vc1dec: Fix null pointer dereference in vc1_decode_skip_blocks()

This handles the last frame being unavailable like all the other
code in vc1dec.

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/vc1dec.c

@@ -4745,9 +4745,11 @@ static void vc1_decode_skip_blocks(VC1Context *v)
         s->mb_x = 0;
         ff_init_block_index(s);
         ff_update_block_index(s);
+        if (s->last_picture.f.data[0]) {
             memcpy(s->dest[0], s->last_picture.f.data[0] + s->mb_y * 16 * s->linesize,   s->linesize   * 16);
             memcpy(s->dest[1], s->last_picture.f.data[1] + s->mb_y *  8 * s->uvlinesize, s->uvlinesize *  8);
             memcpy(s->dest[2], s->last_picture.f.data[2] + s->mb_y *  8 * s->uvlinesize, s->uvlinesize *  8);
+        }
         ff_draw_horiz_band(s, s->mb_y * 16, 16);
         s->first_slice_line = 0;
     }