cli: fix warning about uninitialized value

gcc warns:

    make[4]: Entering directory `./NetworkManager/cli/src'
      CC       connections.o
    connections.c: In function ‘complete_connection_by_type’:
    connections.c:4235:18: error: ‘mtu_int’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
         g_object_set (s_wired, NM_SETTING_WIRED_MTU, mtu_int, NULL);
                      ^

Signed-off-by: Thomas Haller <thaller@redhat.com>

clients/cli/connections.c

@@ -2518,6 +2518,9 @@ check_and_convert_mtu (const char *mtu, guint32 *mtu_int, GError **error)
 {
 	unsigned long local_mtu_int;
 
+	if (mtu_int)
+		*mtu_int = 0;
+
 	if (!mtu)
 		return TRUE;