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devcons: avoid division by zero reading Qsysstat

alexchandel got the kernel to crash with divide error
on qemu 2.1.2/macosx at this location. probably
caused by perfticks()/tsc being wrong or accounttime()
not having been called yet from timer interrupt yet for
some reason.

--- a/sys/src/9/port/devcons.c

+++ b/sys/src/9/port/devcons.c

@@ -581,13 +581,14 @@

 				bp += NUMSIZE;

 				readnum(0, bp, NUMSIZE, mp->load, NUMSIZE);

 				bp += NUMSIZE;

+				l = mp->perf.period;

+				if(l == 0)

+					l = 1;

 				readnum(0, bp, NUMSIZE,

-					(mp->perf.avg_inidle*100)/mp->perf.period,

-					NUMSIZE);

+					(mp->perf.avg_inidle*100)/l, NUMSIZE);

 				bp += NUMSIZE;

 				readnum(0, bp, NUMSIZE,

-					(mp->perf.avg_inintr*100)/mp->perf.period,

-					NUMSIZE);

+					(mp->perf.avg_inintr*100)/l, NUMSIZE);

 				bp += NUMSIZE;

 				*bp++ = '\n';

 			}