vc1dec: require a minimum of 2x2 for the edge pos. Avoid assertion failure

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

libavcodec/vc1dec.c

@@ -5498,6 +5498,8 @@ static int vc1_decode_frame(AVCodecContext *avctx, void *data,
         s->low_delay = !avctx->has_b_frames || v->res_sprite;
 
         if (v->profile == PROFILE_ADVANCED) {
+            if(avctx->coded_width<=1 || avctx->coded_height<=1)
+                goto err;
             s->h_edge_pos = avctx->coded_width;
             s->v_edge_pos = avctx->coded_height;
         }