builtin: add empty string verification for the new string .is_oct() e…

…tc methods, suggested on PR #20540 (#20564)

vlib/builtin/string.v

@@ -1791,6 +1791,10 @@ fn (s string) at_with_check(idx int) ?u8 {
 pub fn (str string) is_oct() bool {
 	mut i := 0
 
+	if str.len == 0 {
+		return false
+	}
+
 	if str[i] == `0` {
 		i++
 	} else if str[i] == `-` || str[i] == `+` {
@@ -1830,6 +1834,10 @@ pub fn (str string) is_oct() bool {
 pub fn (str string) is_bin() bool {
 	mut i := 0
 
+	if str.len == 0 {
+		return false
+	}
+
 	if str[i] == `0` {
 		i++
 	} else if str[i] == `-` || str[i] == `+` {
@@ -1869,6 +1877,10 @@ pub fn (str string) is_bin() bool {
 pub fn (str string) is_hex() bool {
 	mut i := 0
 
+	if str.len == 0 {
+		return false
+	}
+
 	if str[i] == `0` {
 		i++
 	} else if str[i] == `-` || str[i] == `+` {
@@ -1909,6 +1921,10 @@ pub fn (str string) is_hex() bool {
 pub fn (str string) is_int() bool {
 	mut i := 0
 
+	if str.len == 0 {
+		return false
+	}
+
 	if (str[i] != `-` && str[i] != `+`) && (!str[i].is_digit()) {
 		return false
 	} else {

vlib/builtin/string_test.v

@@ -426,6 +426,7 @@ fn test_rsplit_once() ? {
 }
 
 fn test_is_bin() {
+	assert ''.is_bin() == false
 	assert '0b1'.is_bin() == true
 	assert '0b0'.is_bin() == true
 	assert '0b'.is_bin() == false
@@ -455,6 +456,7 @@ fn test_is_bin() {
 }
 
 fn test_is_oct() {
+	assert ''.is_oct() == false
 	assert '0o0'.is_oct() == true
 	assert '0o1'.is_oct() == true
 	assert '0o2'.is_oct() == true
@@ -505,6 +507,7 @@ fn test_is_oct() {
 }
 
 fn test_is_hex() {
+	assert ''.is_hex() == false
 	assert '-324'.is_hex() == false
 	assert '-0'.is_hex() == false
 	assert '0x1'.is_hex() == true
@@ -527,6 +530,7 @@ fn test_is_hex() {
 }
 
 fn test_is_int() {
+	assert ''.is_int() == false
 	assert '-324'.is_int() == true
 	assert '234'.is_int() == true
 	assert '-0'.is_int() == true