vc1dec: Undo mpegvideo initialization if unable to allocate tables

Previously, s->context_initialized was left set to 1
if ff_vc1_decode_init_alloc_tables failed, skipping the
initialization completely on the next decode call.

Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org
Signed-off-by: Martin Storsjö <martin@martin.st>

libavcodec/vc1dec.c

@@ -5877,8 +5877,12 @@ static int vc1_decode_frame(AVCodecContext *avctx, void *data,
     }
 
     if (!s->context_initialized) {
-        if (ff_msmpeg4_decode_init(avctx) < 0 || ff_vc1_decode_init_alloc_tables(v) < 0)
+        if (ff_msmpeg4_decode_init(avctx) < 0)
             goto err;
+        if (ff_vc1_decode_init_alloc_tables(v) < 0) {
+            ff_MPV_common_end(s);
+            goto err;
+        }
 
         s->low_delay = !avctx->has_b_frames || v->res_sprite;