Check dimensions to avoid division by zero due to invalid new dimensions.

Partially fixes http://bugzilla.mplayerhq.hu/show_bug.cgi?id=773.

Originally committed as revision 8321 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/rv10.c

@@ -454,6 +454,8 @@ static int rv20_decode_picture_header(MpegEncContext *s)
         }
         if(new_w != s->width || new_h != s->height){
             av_log(s->avctx, AV_LOG_DEBUG, "attempting to change resolution to %dx%d\n", new_w, new_h);
+            if (avcodec_check_dimensions(s->avctx, new_h, new_w) < 0)
+                return -1;
             MPV_common_end(s);
             s->width  = s->avctx->width = new_w;
             s->height = s->avctx->height= new_h;