Fix uninitialized read in m-tx grep func

[Karl Berry]

Hi Raphael,

    Note that the upstream m-tx repository seems to be written in pascal
    but the problematic version in texlive is (auto translated?) C. That's
    why I sent the patch here. 

Good sleuthing :), but in fact the C code also comes from the upstream
maintainer, Bob Tennent. So I sent your patch to him. (Looks perfectly
sensible to me, FWIW.) --thanks much, karl.