Fix $matches error

Author: staabm

src/Parser/StringUnescaper.php

@@ -2,6 +2,7 @@
 
 namespace PHPStan\PhpDocParser\Parser;
 
+use PHPStan\ShouldNotHappenException;
 use function chr;
 use function hexdec;
 use function octdec;
@@ -56,6 +57,9 @@ static function ($matches) {
 					return chr((int) hexdec(substr($str, 1)));
 				}
 				if ($str[0] === 'u') {
+					if (!isset($matches[2])) {
+						throw new ShouldNotHappenException();
+					}
 					return self::codePointToUtf8((int) hexdec($matches[2]));
 				}