smb3: correct server pointer dereferencing check to be more consistent

Address warning:

    fs/smbfs_client/misc.c:273 header_assemble()
    warn: variable dereferenced before check 'treeCon->ses->server'

Pointed out by Dan Carpenter via smatch code analysis tool

Although the check is likely unneeded, adding it makes the code
more consistent and easier to read, as the same check is
done elsewhere in the function.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>