mk_proxy(): Improve robustness

A lot of code which calls mk_proxy() for DNS resolution assumes that
the "proxy->host->h_addr_list" array contains at least one value, e.g.:

modules/drouting/routing.c +611:
    hostent2ip_addr(&pgw->ips[0], &proxy->host, proxy->addr_idx);

modules/dispatcher/dispatch.c +281:
    hostent2ip_addr( &dp->ips[0], &proxy->host, proxy->addr_idx);

This would cause a segfault immediately if h_addr_list contains 0
results (i.e. first value is a NULL pointer).  As the overall intention
is to resolve the hostname into at least 1 IP address, this patch
changes mk_proxy() to return NULL on a successful query with 0 results.

Credits to Jonathan Hulme for the report

Author: liviuchircu

proxy.c

@@ -213,9 +213,10 @@ struct proxy_l* mk_proxy(str* name, unsigned short port, unsigned short proto,
 	LM_DBG("doing DNS lookup...\n");
 	he = sip_resolvehost(name, &(p->port), &p->proto, is_sips,
 		disable_dns_failover?0:&p->dn );
-	if (he==0){
+	if (!he || !he->h_addr_list[0]) {
 		ser_error=E_BAD_ADDRESS;
-		LM_CRIT("could not resolve hostname: \"%.*s\"\n", name->len, name->s);
+		LM_CRIT("could not resolve hostname: \"%.*s\"%s\n",
+		        name->len, name->s, he ? " (0 results)" : "");
 		pkg_free(p);
 		goto error;
 	}