Infinite loop when removing in empty string.

[mysteryven]

Description

When using a negative index to remove, it will cause infinite loop.

const magicString = require('magic-string');
let s = ""
let ms = new magicString(s);
ms.remove(-2, -1);
console.log(map);

https://stackblitz.com/edit/stackblitz-starters-qxzkza?file=index.js

To reproduce it, run node index.js in the terminal.

Expect behaviour

it should throw error: Character is out of bounds

[antfu]

Would you like to send a quick PR? Thanks!