math.big: fix bytes() if bit_len is divisible by 60, add tests (#25126)

Author: tankf33der

vlib/math/big/big_test.v

@@ -991,3 +991,14 @@ fn test_pow2_is_power_of_2() {
 		assert big.two_int.pow(n).is_power_of_2(), 'pow2: ${n}'
 	}
 }
+
+fn test_bytes_from_bytes() {
+	input := ['0', '9999999999999999999999999999999999999', '783086277830859384',
+		'890810171456467012368983335296321559']
+	for n in input {
+		a := big.integer_from_string(n)!
+		b, _ := a.bytes()
+		c := big.integer_from_bytes(b)
+		assert a.str() == c.str()
+	}
+}

vlib/math/big/integer.v

@@ -891,7 +891,11 @@ pub fn (a Integer) bytes() ([]u8, int) {
 		bits_in_byte = 0
 	}
 	// MSB digit
-	for i := bit_len % digit_bits - 1; i >= 0; i-- {
+	mut msb_bits := bit_len % digit_bits
+	if msb_bits == 0 {
+		msb_bits = digit_bits
+	}
+	for i := msb_bits - 1; i >= 0; i-- {
 		bit = u8((digit >> i) & 1)
 		current_byte = (current_byte << 1) | u8(bit)
 		bits_in_byte++