avfilter/overlay_cuda: fix framesync with embedded PGS subtitle

Signed-off-by: nyanmisaka <nst799610810@gmail.com>

libavfilter/vf_overlay_cuda.c

@@ -157,9 +157,12 @@ static int overlay_cuda_blend(FFFrameSync *fs)
     if (ret < 0)
         return ret;
 
-    if (!input_main || !input_overlay)
+    if (!input_main)
         return AVERROR_BUG;
 
+    if (!input_overlay)
+        return ff_filter_frame(outlink, input_main);
+
     ret = av_frame_make_writable(input_main);
     if (ret < 0) {
         av_frame_free(&input_main);