h263dec: Check for width/height changes on frame skips too.

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

libavcodec/h263dec.c

@@ -438,7 +438,7 @@ retry:
         ret = ff_h263_decode_picture_header(s);
     }
 
-    if (ret < 0) {
+    if (ret < 0 || ret==FRAME_SKIPPED) {
         if (   s->width  != avctx->coded_width
             || s->height != avctx->coded_height) {
                 av_log(s->avctx, AV_LOG_WARNING, "Reverting picture dimensions change due to header decoding failure\n");