math.big: make is_power_of_2() be false for negatives (it now matches Julia's ispow2/1) (#24619)

Author: tankf33der

vlib/math/big/big_test.v

@@ -110,6 +110,7 @@ struct IsXTest {
 
 // vfmt off
 const is_power_of_2_test_data = [
+	IsXTest{ "-4", false },
 	IsXTest{ u32(0b110000000000), false },
 	IsXTest{ "537502395172353242345", false },
 	IsXTest{ "590295810358705700000", false },

vlib/math/big/integer.v

@@ -1214,7 +1214,7 @@ pub fn (x Integer) is_odd() bool {
 // is_power_of_2 returns true when the integer `x` satisfies `2^n`, where `n >= 0`
 @[direct_array_access; inline]
 pub fn (x Integer) is_power_of_2() bool {
-	if x.signum == 0 {
+	if x.signum <= 0 {
 		return false
 	}