troff: fix SIGFPE when using modulus operator

I uncovered this problem while writing unit tests for GNU troff's
delimited expression handling.  Plan 9 troff's numeric expression
evaluator handles division by zero but not modulus by zero.

Fixes:
$ echo '.if %0%0% .tm true' | 9 troff
Floating point exception (core dumped)
$ echo '.if 1%0 .tm true' | 9 troff
Floating point exception (core dumped)

After this patch:
$ echo '.if %0%0% .tm true' | 9 troff
x T utf
x res 720 1 1
x init
troff: modulus by zero.; stdin:1
troff: modulus by zero.; stdin:1
x trailer
V0
x stop
$ echo '.if 1%0 .tm true' | 9 troff
x T utf
x res 720 1 1
x init
troff: modulus by zero.; stdin:1
x trailer
V0
x stop

0 files changed, 0 insertions, 0 deletions