config: a user-provided invalid section is not a BUG

This was pointed out by Jeff King while the empty-config-section-fix
patch series was cooking, and was not addressed in time for that patch
series to advance to `master`.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

config.c

@@ -2329,7 +2329,7 @@ static int store_aux_event(enum config_event_t type,
 
 	if (type == CONFIG_EVENT_SECTION) {
 		if (cf->var.len < 2 || cf->var.buf[cf->var.len - 1] != '.')
-			BUG("Invalid section name '%s'", cf->var.buf);
+			return error("invalid section name '%s'", cf->var.buf);
 
 		/* Is this the section we were looking for? */
 		store->is_keys_section =