fix pistatistic mean calc

libs/main/math/pistatistic.h

@@ -46,8 +46,8 @@ public:
 	bool calculate(const PIVector<T> & val, const T & given_mean) {
 		T v = T(), v1 = T(), v2 = T(), stddev = T(), var = T();
 		int i, n = val.size();
-		if (n < 2) return false;
 		mean     = given_mean;
+		if (n < 2) return false;
 		variance = skewness = kurtosis = T();
 		// Variance (using corrected two-pass algorithm)
 		for (i = 0; i < n; i++)