res_pjsip_registrar: Fix uninitlized variable warning

Fixes: error: ‘domain_name’ may be used uninitialized in this function

Found with gcc (Ubuntu 9.2.1-9ubuntu2) 9.2.1 20191008

Change-Id: I44413b49ea1205aa25538142161deb73883c79e8

res/res_pjsip_registrar.c

@@ -1017,7 +1017,7 @@ static struct ast_sip_aor *find_registrar_aor(struct pjsip_rx_data *rdata, struc
 {
 	struct ast_sip_aor *aor = NULL;
 	char *aor_name = NULL;
-	char *domain_name;
+	char *domain_name = NULL;
 	char *username = NULL;
 	int i;