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>