cavsdec: check for changing w/h.

Our decoder does not support changing w/h.

Fixes CVE-2012-2777 and CVE-2012-2784.

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavcodec/cavsdec.c

@@ -1056,12 +1056,21 @@ static int decode_pic(AVSContext *h) {
 static int decode_seq_header(AVSContext *h) {
     MpegEncContext *s = &h->s;
     int frame_rate_code;
+    int width, height;
 
     h->profile =         get_bits(&s->gb,8);
     h->level =           get_bits(&s->gb,8);
     skip_bits1(&s->gb); //progressive sequence
-    s->width =           get_bits(&s->gb,14);
-    s->height =          get_bits(&s->gb,14);
+
+    width  = get_bits(&s->gb, 14);
+    height = get_bits(&s->gb, 14);
+    if ((s->width || s->height) && (s->width != width || s->height != height)) {
+        av_log_missing_feature(s, "Width/height changing in CAVS is", 0);
+        return AVERROR_PATCHWELCOME;
+    }
+    s->width  = width;
+    s->height = height;
+
     skip_bits(&s->gb,2); //chroma format
     skip_bits(&s->gb,3); //sample_precision
     h->aspect_ratio =    get_bits(&s->gb,4);