AbstractClassRestrictionsSniff: fix insufficient defensive coding

This commit fixes the test failure seen in PR 2499.

The test failure was surfaced due to a new exception in PHPCS (see: PHPCSStandards/PHP_CodeSniffer 524), which will be included in PHPCS 3.11.0.

The test failure highlighted that the above mentioned PHPCS PR needs a follow-up with some extra defensive coding, however, that defensive coding is needed in a part in PHPCS which does the error handling for PHP notices being encountered, i.e. the throwing of `Internal.Exception` errors.

This implied there was also an underlying issue in WPCS causing the `Internal.Exception`, which apparently was only triggered when the fixer was being run.

> Note: the fact that `Internal.Exception`s during a PHPCBF run are being suppressed is a known issue, but fixing that is not that easy, so that definitely won't happen before PHPCS 4.0 (and may not even make it into 4.0). See: squizlabs/PHP_CodeSniffer 2871

Either way, I've looked into the `Internal.Exception` now.
The error was as follows:
```
An error occurred during processing; checking has been aborted. The error message was: PHPCSUtils\Utils\GetTokensAsString::getString(): Argument #2 ($start) must be a stack pointer which exists in the $phpcsFile object, 8 given.
The error originated in the AbstractClassRestrictionsSniff.php sniff on line 131.
```

The additional defensive coding added in this commit, fixes the issue.

WordPress/AbstractClassRestrictionsSniff.php

@@ -128,8 +128,10 @@ public function is_targetted_token( $stackPtr ) {
 				$nameEnd = ( $this->phpcsFile->findNext( array( \T_CLOSE_CURLY_BRACKET, \T_WHITESPACE ), ( $stackPtr + 2 ) ) - 1 );
 			}
 
-			$classname = GetTokensAsString::noEmpties( $this->phpcsFile, ( $stackPtr + 2 ), $nameEnd );
-			$classname = $this->get_namespaced_classname( $classname, ( $stackPtr - 1 ) );
+			if ( isset( $this->tokens[ $stackPtr + 2 ] ) && false !== $nameEnd ) {
+				$classname = GetTokensAsString::noEmpties( $this->phpcsFile, ( $stackPtr + 2 ), $nameEnd );
+				$classname = $this->get_namespaced_classname( $classname, ( $stackPtr - 1 ) );
+			}
 		}
 
 		if ( \T_DOUBLE_COLON === $token['code'] ) {