vf_overlay_opencl: Don't leak output frame on error

Fixes CID #1423277.

libavfilter/vf_overlay_opencl.c

@@ -250,6 +250,7 @@ fail_kernel_arg:
            kernel_arg, cle);
     err = AVERROR(EIO);
 fail:
+    av_frame_free(&output);
     return err;
 }