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/truemotion1.c

@@ -474,6 +474,7 @@ static av_cold int truemotion1_decode_init(AVCodecContext *avctx)
 //    else
 //        avctx->pix_fmt = PIX_FMT_RGB555;
 
+    avcodec_get_frame_defaults(&s->frame);
     s->frame.data[0] = NULL;
 
     /* there is a vertical predictor for each pixel in a line; each vertical