Squiz/OperatorBracket: prevent PHP notices during live coding

During live coding (or in the case of parse errors), the "end of the expression" cannot always be correctly determined. In such a case, the sniff should stay silent.

This wasn't always handled correctly so far and could lead to the following PHP notices:
```
Undefined array key "parenthesis_closer" in path/to/phpcs/src/Standards/Squiz/Sniffs/Formatting/OperatorBracketSniff.php on line 357
Undefined array key "bracket_closer" in path/to/phpcs/src/Standards/Squiz/Sniffs/Formatting/OperatorBracketSniff.php on line 362
```

This commit fixes these by adding some extra defensive coding and bowing out when an unclosed bracket set is encountered.

Includes tests.

Author: jrfnl

src/Standards/Squiz/Sniffs/Formatting/OperatorBracketSniff.php

@@ -354,16 +354,23 @@ public function addMissingBracketsError($phpcsFile, $stackPtr)
             }
 
             if ($tokens[$after]['code'] === T_OPEN_PARENTHESIS) {
+                if (isset($tokens[$after]['parenthesis_closer']) === false) {
+                    // Live coding/parse error. Ignore.
+                    return;
+                }
+
                 $after = $tokens[$after]['parenthesis_closer'];
                 continue;
             }
 
-            if ($tokens[$after]['code'] === T_OPEN_SQUARE_BRACKET) {
-                $after = $tokens[$after]['bracket_closer'];
-                continue;
-            }
+            if (($tokens[$after]['code'] === T_OPEN_SQUARE_BRACKET
+                || $tokens[$after]['code'] === T_OPEN_SHORT_ARRAY)
+            ) {
+                if (isset($tokens[$after]['bracket_closer']) === false) {
+                    // Live coding/parse error. Ignore.
+                    return;
+                }
 
-            if ($tokens[$after]['code'] === T_OPEN_SHORT_ARRAY) {
                 $after = $tokens[$after]['bracket_closer'];
                 continue;
             }

src/Standards/Squiz/Tests/Formatting/OperatorBracketUnitTest.2.inc

@@ -0,0 +1,7 @@
+<?php
+
+// Intentional parse error. This has to be the last (and only) test in the file.
+// Live coding test. The sniff should stay silent.
+class ParseErrors {
+    const A|(B PARSE_ERROR = null;
+}

src/Standards/Squiz/Tests/Formatting/OperatorBracketUnitTest.3.inc

@@ -0,0 +1,5 @@
+<?php
+
+// Intentional parse error. This has to be the last (and only) test in the file.
+// Live coding test. The sniff should stay silent.
+$a = [10, 20] + [