avcodec/vp9: Rollback dimensions when format is rejected

Fixes: BMO#2029296

Found-by: Mozilla Security Team, Paul Adenot for the write variant
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 6c114bd6fa)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/vp9.c

@@ -244,8 +244,10 @@ static int update_size(AVCodecContext *avctx, int w, int h)
         *fmtp = AV_PIX_FMT_NONE;
 
         ret = ff_thread_get_format(avctx, pix_fmts);
-        if (ret < 0)
+        if (ret < 0) {
+            ff_set_dimensions(avctx, s->w, s->h);
             return ret;
+        }
 
         avctx->pix_fmt = ret;
         s->gf_fmt  = s->pix_fmt;