urlmatch.c: add and use a *_release() function

Plug a memory leak in credential_apply_config() by adding and using a
new urlmatch_config_release() function. This just does a
string_list_clear() on the "vars" member.

This finished up work on normalizing the init/free pattern in this
API, started in 73ee449bbf (urlmatch.[ch]: add and use
URLMATCH_CONFIG_INIT, 2021-10-01).

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

urlmatch.c

@@ -611,3 +611,8 @@ int urlmatch_config_entry(const char *var, const char *value, void *cb)
 	strbuf_release(&synthkey);
 	return retval;
 }
+
+void urlmatch_config_release(struct urlmatch_config *config)
+{
+	string_list_clear(&config->vars, 1);
+}