h263dec: Disallow width/height changing with frame threads.

Fixes CVE-2011-3937

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

libavcodec/h263dec.c

@@ -570,8 +570,7 @@ retry:
 #if HAVE_MMX
     if (s->codec_id == CODEC_ID_MPEG4 && s->xvid_build>=0 && avctx->idct_algo == FF_IDCT_AUTO && (av_get_cpu_flags() & AV_CPU_FLAG_MMX)) {
         avctx->idct_algo= FF_IDCT_XVIDMMX;
-        avctx->coded_width= 0; // force reinit
-//        dsputil_init(&s->dsp, avctx);
+        ff_dct_common_init(s);
         s->picture_number=0;
     }
 #endif
@@ -585,6 +584,12 @@ retry:
         || s->height != avctx->coded_height) {
         /* H.263 could change picture size any time */
         ParseContext pc= s->parse_context; //FIXME move these demuxng hack to avformat
+
+        if (HAVE_THREADS && (s->avctx->active_thread_type&FF_THREAD_FRAME)) {
+            av_log_missing_feature(s->avctx, "Width/height/bit depth/chroma idc changing with threads is", 0);
+            return -1;   // width / height changed during parallelized decoding
+        }
+
         s->parse_context.buffer=0;
         MPV_common_end(s);
         s->parse_context= pc;