Fix typo.

Author: Andriy Pylypenko

resolve.c

@@ -267,7 +267,7 @@ struct hostent * own_gethostbyaddr(void *addr, socklen_t len, int af)
 
 query:
 	cached_he = gethostbyaddr(addr, len, af);
-	if (cached_he < 0) {
+	if (cached_he == NULL) {
 		LM_DBG("ptr not found\n");
 		if (dnscache_put_func(addr,T_PTR,NULL,len,1,0) < 0)
 			LM_ERR("Failed to store PTR in cache\n");