avfilter/vf_paletteuse: enable skip_initial_unpaired

Fixes crash due to unprocessed input being passed through

This fixes the last segfault caused by mixing 3.0 and 3.1 libs and
applications
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavfilter/vf_paletteuse.c

@@ -1003,6 +1003,7 @@ static av_cold int init(AVFilterContext *ctx)
 {
     PaletteUseContext *s = ctx->priv;
     s->dinput.repeatlast = 1; // only 1 frame in the palette
+    s->dinput.skip_initial_unpaired = 1;
     s->dinput.process    = load_apply_palette;
 
     s->set_frame = set_frame_lut[s->color_search_method][s->dither];