Fix crash with snapshot testing

Seems like `String#repeat` does not like negative numbers.

lib/code-excerpt.js

@@ -6,7 +6,7 @@ const truncate = require('cli-truncate');
 const chalk = require('chalk');
 
 const formatLineNumber = (lineNumber, maxLineNumber) =>
-	' '.repeat(String(maxLineNumber).length - String(lineNumber).length) + lineNumber;
+	' '.repeat(Math.max(0, String(maxLineNumber).length - String(lineNumber).length)) + lineNumber;
 
 module.exports = (file, line, options) => {
 	options = options || {};