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

src/cmd/troff/n4.c

@@ -453,6 +453,11 @@ a0:
 		i = ckph();
 		if (nonumb)
 			break;
+		if (i == 0) {
+			flusho();
+			ERROR "modulus by zero." WARN;
+			acc = 0;
+		} else
 			acc %= i;
 		goto a0;
 	case '&':	/*and*/