avcodec/vc1dec: Postpone allocating sprite frame to avoid segfault

Up until now, the VC-1 decoders allocated an AVFrame for usage with
sprites during vc1_decode_init(); yet said AVFrame can be freed if
(re)initializing the context (which happens ordinarily during decoding)
fails. The AVFrame does not get allocated again lateron in this case,
leading to segfaults.

Fix this by moving the allocation of said frame immediately before it is
used (this also means that said frame won't be allocated at all any more
in case of a regular (i.e. non-image) stream).

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavcodec/vc1dec.c

@@ -539,12 +539,6 @@ static av_cold int vc1_decode_init(AVCodecContext *avctx)
     ff_h264chroma_init(&v->h264chroma, 8);
     ff_qpeldsp_init(&s->qdsp);
 
-    // Must happen after calling ff_vc1_decode_end
-    // to avoid de-allocating the sprite_output_frame
-    v->sprite_output_frame = av_frame_alloc();
-    if (!v->sprite_output_frame)
-        return AVERROR(ENOMEM);
-
     avctx->has_b_frames = !!avctx->max_b_frames;
 
     if (v->color_prim == 1 || v->color_prim == 5 || v->color_prim == 6)
@@ -577,20 +571,15 @@ static av_cold int vc1_decode_init(AVCodecContext *avctx)
             v->sprite_height > 1 << 14 ||
             v->output_width  > 1 << 14 ||
             v->output_height > 1 << 14) {
-            ret = AVERROR_INVALIDDATA;
-            goto error;
+            return AVERROR_INVALIDDATA;
         }
 
         if ((v->sprite_width&1) || (v->sprite_height&1)) {
             avpriv_request_sample(avctx, "odd sprites support");
-            ret = AVERROR_PATCHWELCOME;
-            goto error;
+            return AVERROR_PATCHWELCOME;
         }
     }
     return 0;
-error:
-    av_frame_free(&v->sprite_output_frame);
-    return ret;
 }
 
 /** Close a VC1/WMV3 decoder
@@ -1147,6 +1136,11 @@ image:
         avctx->height = avctx->coded_height = v->output_height;
         if (avctx->skip_frame >= AVDISCARD_NONREF)
             goto end;
+        if (!v->sprite_output_frame &&
+            !(v->sprite_output_frame = av_frame_alloc())) {
+            ret = AVERROR(ENOMEM);
+            goto err;
+        }
 #if CONFIG_WMV3IMAGE_DECODER || CONFIG_VC1IMAGE_DECODER
         if ((ret = vc1_decode_sprites(v, &s->gb)) < 0)
             goto err;