h261dec: Fix context initialization sequence

ff_mpv_common_init sets s->context_initialized.

This fixes decoding of h261 in the cases where the demuxer
hasn't already set the frame size.

CC: libav-stable@libav.org
Signed-off-by: Martin Storsjö <martin@martin.st>
(cherry picked from commit 3bb465245f)
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

libavcodec/h261dec.c

@@ -600,11 +600,10 @@ retry:
         s->parse_context = pc;
     }
 
-    if (!s->context_initialized)
+    if (!s->context_initialized) {
         if ((ret = ff_mpv_common_init(s)) < 0)
             return ret;
 
-    if (!s->context_initialized) {
         ret = ff_set_dimensions(avctx, s->width, s->height);
         if (ret < 0)
             return ret;