avfilter/palettegen: make sure at least one frame was sent to the filter

Fix FPE.

libavfilter/vf_palettegen.c

@@ -504,7 +504,7 @@ static int request_frame(AVFilterLink *outlink)
     int r;
 
     r = ff_request_frame(inlink);
-    if (r == AVERROR_EOF && !s->palette_pushed) {
+    if (r == AVERROR_EOF && !s->palette_pushed && s->nb_refs) {
         r = ff_filter_frame(outlink, get_palette_frame(ctx));
         s->palette_pushed = 1;
         return r;