Squiz/DisallowMultipleAssignments: fix false positive

There was a bug in the code used to check if a equal sign is used to set
a default value for a parameter that caused a false positive when the
function was missing the closing parenthesis (which can happen during
live coding). This commit fixes this false positive.

Author: rodrigoprimo

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

@@ -47,7 +47,7 @@ public function process(File $phpcsFile, $stackPtr)
         if ($function !== false) {
             $opener = $tokens[$function]['parenthesis_opener'];
             $closer = $tokens[$function]['parenthesis_closer'];
-            if ($opener < $stackPtr && $closer > $stackPtr) {
+            if ($closer === null || ($opener < $stackPtr && $closer > $stackPtr)) {
                 return;
             }
         }

src/Standards/Squiz/Tests/PHP/DisallowMultipleAssignmentsUnitTest.2.inc

@@ -0,0 +1,7 @@
+<?php
+
+// Intentional parse error (missing closing parenthesis).
+// This should be the only test in this file.
+// Testing that the sniff is *not* triggered.
+
+function missingClosingParenthesis($a =