Hi,

Enclosed is a patch to handle roff.c's division by zero.  This was found 
(indirectly) when playing with mandoc's new support for inline equations.

groff(1) seems to actually warn us when this happens--if we're to do the 
same, we'll need to pass the mparse function through to the offending 
function.

By the way, upon an audit, I see that term_ps.c is vulnerable to 
division by zero if the "scale" parameter in termp_ps is zero.  However, 
we don't ever seem to re-set that, so it's ok.

Best,

Kristaps