math: fix error for math.abs(0.0)/math.abs(0) (related #14165) (#14191)

vlib/math/math_test.v

@@ -408,6 +408,16 @@ fn test_abs() {
 	}
 }
 
+fn test_abs_zero() {
+	ret1 := abs(0)
+	println(ret1)
+	assert '$ret1' == '0'
+
+	ret2 := abs(0.0)
+	println(ret2)
+	assert '$ret2' == '0'
+}
+
 fn test_floor() {
 	for i := 0; i < math.vf_.len; i++ {
 		f := floor(math.vf_[i])

vlib/math/mathutil.v

@@ -18,5 +18,5 @@ pub fn max<T>(a T, b T) T {
 // abs returns the absolute value of `a`
 [inline]
 pub fn abs<T>(a T) T {
-	return if a > 0 { a } else { -a }
+	return if a < 0 { -a } else { a }
 }