list-objects-filter: plug combine_filter_data leak

filter_combine__init() allocates a struct combine_filter_data object and
assigns it to the filter_data member of struct filter_options.  Release
it in the complementing filter_combine__free().

Reported-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

list-objects-filter.c

@@ -709,6 +709,7 @@ static void filter_combine__free(void *filter_data)
 			BUG("expected oidset to be cleared already");
 	}
 	free(d->sub);
+	free(d);
 }
 
 static void add_all(struct oidset *dest, struct oidset *src) {