Call getnameinfo with NI_NAMEREQD flag.

This way net_gethostbyaddr will fail when the system is unable to
resolve the address to a valid host name.
Without this flag in case of failure the function would return the ip
address instead of failing.

src/core/network.c

@@ -448,7 +448,9 @@ int net_gethostbyaddr(IPADDR *ip, char **name)
 
 	/* save error to host_error for later use */
         host_error = getnameinfo((struct sockaddr *)&so, sizeof(so),
-                                 hostname, sizeof(hostname), NULL, 0, 0);
+				 hostname, sizeof(hostname),
+				 NULL, 0,
+				 NI_NAMEREQD);
         if (host_error != 0)
                 return host_error;