string: do not free empty strings

vlib/builtin/string.v

@@ -1183,7 +1183,9 @@ pub fn (c byte) is_letter() bool {
 }
 
 pub fn (s &string) free() {
-	if s.is_lit {return}
+	if s.is_lit || s.len == 0 {
+		return
+	}
 	free(s.str)
 }