h264: don't initialize missing pictures when using a hwaccel

Writing into uninitialized hw surfaces is not supported and triggers an assert inside avpriv_color_frame
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/h264.c

@@ -1704,7 +1704,7 @@ int ff_h264_frame_start(H264Context *h)
 
     if ((ret = alloc_picture(h, pic)) < 0)
         return ret;
-    if(!h->sync)
+    if(!h->sync && !h->avctx->hwaccel)
         avpriv_color_frame(&pic->f, c);
 
     h->cur_pic_ptr = pic;