sock_util: Add terminator to host-port string

Add missing null terminator to the returned host-port string in
sock_udp_ep_fmt

sys/net/sock/sock_util.c

@@ -133,6 +133,7 @@ int sock_urlsplit(const char *url, char *hostport, char *urlpath)
         return -EOVERFLOW;
     }
     memcpy(hostport, hoststart, hostlen);
+    *(hostport + hostlen) = '\0';
 
     size_t pathlen = strlen(pathstart);
     if (pathlen) {