bug symfony#21401 [Debug] Workaround "null" $context (nicolas-grekas)

This PR was merged into the 2.7 branch.

Discussion
----------

[Debug] Workaround "null" $context

| Q             | A
| ------------- | ---
| Branch?       | 2.7
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

As reported in symfony#20068, the imap extension can call error handlers with $context = null.

Commits
-------

2555f31 [Debug] Workaround "null" $context

Author: fabpot

src/Symfony/Component/Debug/ErrorHandler.php

@@ -373,7 +373,7 @@ public function handleError($type, $message, $file, $line)
         $scope = $this->scopedErrors & $type;
 
         if (4 < $numArgs = func_num_args()) {
-            $context = $scope ? func_get_arg(4) : array();
+            $context = $scope ? (func_get_arg(4) ?: array()) : array();
             $backtrace = 5 < $numArgs ? func_get_arg(5) : null; // defined on HHVM
         } else {
             $context = array();