games/frobtads: unbreak with clang 4.0

tads3/vmtz.cpp:1987:30: error: ordered comparison between pointer and zero ('vmtz_trans *' and 'int')

result->set(tcur > 0 ? tcur - 1 : tcur);
            ~~~~ ^ ~

PR: 216357
Reported by: antoine (via exp-run)
Submitted by: kaiwang27@gmail.com (maintainer)