CID 451444: division by zero in lib/isc/histo.c

Coverity Scan claims division by zero in lib/isc/histo.c, introduced in !7696 (merged).

/lib/isc/histo.c: 477 in isc_histo_moments()
471     		mean += count * delta / pop;
472     		sigma += count * delta * (value - mean);
473     	}
474     
475     	OUTARG(pm0, pop);
476     	OUTARG(pm1, mean);
>>>     CID 451444:  Incorrect expression  (DIVIDE_BY_ZERO)
>>>     In expression "sigma / pop", division by expression "pop" which may be zero has undefined behavior.
477     	OUTARG(pm2, sqrt(sigma / pop));
478     }
479     
480     /*
481      * Clamped linear interpolation
482      *