svq1dec: call avcodec_set_dimensions() after dimensions changed.

Fixes NGS00148, CVE-2011-4579

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

libavcodec/svq1dec.c

@@ -659,6 +659,7 @@ static int svq1_decode_frame(AVCodecContext *avctx,
     av_dlog(s->avctx, "Error in svq1_decode_frame_header %i\n",result);
     return result;
   }
+  avcodec_set_dimensions(avctx, s->width, s->height);
 
   //FIXME this avoids some confusion for "B frames" without 2 references
   //this should be removed after libavcodec can handle more flexible picture types & ordering