strconv: fix fxx_to_str_l_parse/fxx_to_str_l_parse_no_dot returning s without cloning

vlib/builtin/array_test.v

@@ -1092,8 +1092,6 @@ fn test_array_int_pop() {
 	assert z == 4
 	x1 := a.pop()
 	x2 := a.pop()
-	dump(x1)
-	dump(x2)
 	final := a.pop()
 	assert final == 1
 }

vlib/builtin/string_int_test.v

@@ -186,7 +186,7 @@ fn test_signed_cast() {
 	}
 
 	// test g format
-	{
+	unsafe {
 		mut u := strconv.Float64u{
 			u: strconv.double_plus_zero
 		}
@@ -202,7 +202,7 @@ fn test_signed_cast() {
 		assert '${u.f:g}' == '-inf'
 		assert '${u.f:G}' == '-INF'
 	}
-	{
+	unsafe {
 		mut u := strconv.Float32u{
 			u: strconv.single_plus_zero
 		}

vlib/strconv/utilities.v

@@ -272,7 +272,7 @@ pub fn f64_to_str_l_no_dot(f f64) string {
 pub fn fxx_to_str_l_parse(s string) string {
 	// check for +inf -inf Nan
 	if s.len > 2 && (s[0] == `n` || s[1] == `i`) {
-		return s
+		return s.clone()
 	}
 
 	m_sgn_flag := false
@@ -395,7 +395,7 @@ pub fn fxx_to_str_l_parse(s string) string {
 pub fn fxx_to_str_l_parse_no_dot(s string) string {
 	// check for +inf -inf Nan
 	if s.len > 2 && (s[0] == `n` || s[1] == `i`) {
-		return s
+		return s.clone()
 	}
 
 	m_sgn_flag := false