assert that no other types than TGOPER can happen in SmParser::Oper()

TOVERBRACE/TUNDERBRACE belongs to TGPRODUCT.

Change-Id: I34b23858fb70f38ffd82e373e16ab68632d6e303
Reviewed-on: https://gerrit.libreoffice.org/12508
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>