Fix possible index error in NonExecutableCodeSniff

I was running into this while working on another sniff. One trivial example I found to demonstrate the bug is `<?php return array_map(`. This can happen when PHPCS runs on a file I'm currently typing in, but the file is not complete yet.

I believe it's safe to assume the 'parenthesis_closer' array key only exists on T_OPEN_PARENTHESIS tokens, and the 'bracket_closer' key only on T_OPEN_CURLY_BRACKET tokens. I had a quick look at the tokenizer and I believe this is indeed the case. But I might be wrong.

Sorry for not providing a test. I would love to, but I'm not familiar enough with the test setup here.

src/Standards/Squiz/Sniffs/PHP/NonExecutableCodeSniff.php

@@ -211,12 +211,12 @@ public function process(File $phpcsFile, $stackPtr)
                 break;
             }
 
-            if ($tokens[$start]['code'] === T_OPEN_PARENTHESIS) {
+            if (isset($tokens[$start]['parenthesis_closer']) === true) {
                 $start = $tokens[$start]['parenthesis_closer'];
                 continue;
             }
 
-            if ($tokens[$start]['code'] === T_OPEN_CURLY_BRACKET) {
+            if (isset($tokens[$start]['bracket_closer']) === true) {
                 $start = $tokens[$start]['bracket_closer'];
                 continue;
             }