h264: allocate hwaccel privdata after the frame buffer

This ensures the hwaccel privdata does not leak when a frame buffer could
not be allocated (and toggle the assert when the frame is re-used).
Having no frame buffer available is quite common when using the DXVA2
hwaccel in situations where the DXVA2 renderer is being re-allocated, for
example when moving between displays.
Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavcodec/h264.c

@@ -318,6 +318,15 @@ static int alloc_picture(H264Context *h, Picture *pic)
 
     av_assert0(!pic->f.data[0]);
 
+    pic->tf.f = &pic->f;
+    ret = ff_thread_get_buffer(h->avctx, &pic->tf, pic->reference ?
+                                                   AV_GET_BUFFER_FLAG_REF : 0);
+    if (ret < 0)
+        goto fail;
+
+    h->linesize   = pic->f.linesize[0];
+    h->uvlinesize = pic->f.linesize[1];
+
     if (h->avctx->hwaccel) {
         const AVHWAccel *hwaccel = h->avctx->hwaccel;
         av_assert0(!pic->hwaccel_picture_private);
@@ -328,14 +337,6 @@ static int alloc_picture(H264Context *h, Picture *pic)
             pic->hwaccel_picture_private = pic->hwaccel_priv_buf->data;
         }
     }
-    pic->tf.f = &pic->f;
-    ret = ff_thread_get_buffer(h->avctx, &pic->tf, pic->reference ?
-                                                   AV_GET_BUFFER_FLAG_REF : 0);
-    if (ret < 0)
-        goto fail;
-
-    h->linesize   = pic->f.linesize[0];
-    h->uvlinesize = pic->f.linesize[1];
 
     if (!h->qscale_table_pool) {
         ret = init_table_pools(h);