fix a string bug that occurs when using both dshell and Json module

Fix a string bug that occurs when using both a large script (such as Syntax.ShellStyle package) and Json module.
If over 16 bytes of string is passed to json value, it becomes incorrect string.

## input
```
Import("Syntax.ShellStyle");
Import("Type.Json");

void test() {
	Json j = new Json();
	j.setString("16bytes", "0123456789012345");
	System.p(j.getString("16bytes"));
	j.setString("17bytes", "01234567890123456");
	System.p(j.getString("17bytes"));
}

test();
```

## output
```
 - (json.k:7) 0123456789012345
 - (json.k:9) 01234567890123456r/sbin:/sbin
```