tls_mgm: fix crash when the client_sip_domain_avp AVP is not set

This is a regression introduced by commit e94ca5f that would lead to
a crash when the client_sip_domain_avp modparam is set but the actual
AVP is not set before a TLS connect.

Credits to @ovidiusas for reporting and suggesting the fix.

Closes #2909

(cherry picked from commit 3c05ae7)

modules/tls_mgm/tls_domain.c

@@ -475,6 +475,8 @@ struct tls_domain *tls_find_client_domain(struct ip_addr *ip, unsigned short por
 			if (sip_domain) {
 				LM_DBG("Match TLS domain by sip domain AVP: '%.*s'\n",
 					sip_domain->len, ZSW(sip_domain->s));
+			} else {
+				sip_domain = &match_any_dom;
 			}
 		}
 	} else {