smb3: check for null tcon

Although unlikely to be null, it is confusing to use a pointer
before checking for it to be null so move the use down after
null check.

Addresses-Coverity: 1517586 ("Null pointer dereferences  (REVERSE_INULL)")
Reviewed-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>