vf_paletteuse: Don't free the second frame from ff_framesync_dualinput_get_writable on error

This fixes a double free in he error case.
Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>

libavfilter/vf_paletteuse.c

@@ -1037,7 +1037,6 @@ static int load_apply_palette(FFFrameSync *fs)
 
 error:
     av_frame_free(&master);
-    av_frame_free(&second);
     return ret;
 }