libopenjpeg: set internal frame defaults

Fix ffmpeg decoding, which was broken after the recent changes in
vsrc_buffer.

libavcodec/libopenjpeg.c

@@ -53,6 +53,7 @@ static av_cold int libopenjpeg_decode_init(AVCodecContext *avctx)
     LibOpenJPEGContext *ctx = avctx->priv_data;
 
     opj_set_default_decoder_parameters(&ctx->dec_params);
+    avcodec_get_frame_defaults(&ctx->image);
     avctx->coded_frame = &ctx->image;
     return 0;
 }