Let the standard deviation of the empty set be 0. This is consistent with

the behaviour for averages, and fixes a crash that can occur when attempting
to print a stddev aggregation containing no elements:

    dtrace:::BEGIN { @ = stddev(0); clear(@); printa("%@d", @); }

PR:		197260
MFC after:	2 week

cddl/contrib/opensolaris/lib/libdtrace/common/dt_consume.c

@@ -382,6 +382,9 @@ dt_stddev(uint64_t *data, uint64_t normal)
 	int64_t norm_avg;
 	uint64_t diff[2];
 
+	if (data[0] == 0)
+		return (0);
+
 	/*
 	 * The standard approximation for standard deviation is
 	 * sqrt(average(x**2) - average(x)**2), i.e. the square root