Possible SEGV (null pointer deref) in writet1.c

[Karl Berry]

    It is worth noting that there are three variants (sic!) of 'writet1.c' 
    in the TeX Live source tree with idential code around 'strstr' and 
    'strlen'. I suggest to fix all of them just to be on the safe side.

Thanks for the reminder, Andreas. I will.

(I used to have an automatic merge of the dvips and pdftex versions, but
that became untenable at some point. Sigh ...)