Generic/JumbledIncrementer: fix non-problematic bug

The sniff was wrongly assuming that the inner for loop would always have
a closing parenthesis which is not true when PHPCS is used with live
coding. This meant that `$end` could be set to `null`
(https://github.com/PHPCSStandards/PHP_CodeSniffer/blob/84acf4e56f110db8e75cb9a575c5727df637643c/src/Standards/Generic/Sniffs/CodeAnalysis/JumbledIncrementerSniff.php#L116). This did not cause any issues as `$next <= $end`
when `$end` is `null` is always false, so the for loop that looks for
incrementers was not executed.

The code was changed to bail early when the inner for loop is missing
the closing parenthesis and a test was added.

Author: rodrigoprimo

src/Standards/Generic/Sniffs/CodeAnalysis/JumbledIncrementerSniff.php

@@ -108,7 +108,7 @@ public function process(File $phpcsFile, $stackPtr)
     protected function findIncrementers(array $tokens, array $token)
     {
         // Skip invalid statement.
-        if (isset($token['parenthesis_opener']) === false) {
+        if (isset($token['parenthesis_opener'], $token['parenthesis_closer']) === false) {
             return [];
         }
 

src/Standards/Generic/Tests/CodeAnalysis/JumbledIncrementerUnitTest.2.inc

@@ -0,0 +1,8 @@
+<?php
+
+// Intentional parse error (inner for loop missing closing parenthesis).
+// This should be the only test in this file.
+// Testing that the sniff is *not* triggered.
+for ($i = 0; $i < 20; $i++) {
+    for ($i = 0; $i < 20; $i++
+}