lavc: set defaults in internal codec frames

This is required specifically for setting frame->format to -1,
otherwise it will be set to 0 = PIX_FMT_YUV420P and code reading
the format from the output decoded frame will get misled.

In particular fix regressions occurring with the pending vsrc_buffer
patch.

libavcodec/flicvideo.c

@@ -118,6 +118,7 @@ static av_cold int flic_decode_init(AVCodecContext *avctx)
                   return -1;
     }
 
+    avcodec_get_frame_defaults(&s->frame);
     s->frame.data[0] = NULL;
     s->new_palette = 0;