cifs: Avoid error pointer dereference

The patch 7d6535b7: "cifs: Simplify reconnect code when dfs
upcall is enabled" leads to the following static checker warning:

	fs/cifs/connect.c:160 reconn_set_next_dfs_target()
	error: 'server->hostname' dereferencing possible ERR_PTR()

Avoid dereferencing the error pointer by early returning on error
condition.
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Samuel Cabrero <scabrero@suse.de>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/cifs/connect.c

@@ -155,6 +155,7 @@ static void reconn_set_next_dfs_target(struct TCP_Server_Info *server,
 		cifs_dbg(FYI,
 			 "%s: failed to extract hostname from target: %ld\n",
 			 __func__, PTR_ERR(server->hostname));
+		return;
 	}
 
 	rc = reconn_set_ipaddr_from_hostname(server);