Reset client DCB pointer after closing it

The client DCB was not reset in one of the cases where it was closed.

Also fixed use of overlapping memory addresses in strcpy.

server/modules/routing/binlogrouter/blr_master.c

@@ -340,6 +340,7 @@ static void
 blr_restart_master(ROUTER_INSTANCE *router)
 {
     dcb_close(router->client);
+    router->client = NULL;
 
     /* Now it is safe to unleash other threads on this router instance */
     spinlock_acquire(&router->lock);