[tex-k] Total number of words in pltotf

[Karl Berry]

    This conversion mistakenly allows 'lf' to overflow, and it ends
    up generating invalid TFM file. 

I suppose we could somehow make it convert to unsigned short, but that
wouldn't help in general, since it could still be longer than 16
bits. And trying to use uint16_t will surely cause endless compatibility
problems to ensue, so forget that.

    I hope the attached patch can be safely applied to pltotf.ch.

It looks fine to me. In principle it would be nice to include the
erroneous value in the error message, but since TeX programs are usually
lacking in that regard, it's no biggie.

Thanks,
Karl