Fix "Invalid argument supplied for foreach()" on a new PlaformRepository obj

This happens on "new PlatformRepository(array($somePackage))".
The parent constructor calls "\Composer\Repository\ArrayRepository::addPackage()",
which, on a brand new repo object, further calls "\Composer\Repository\PlatformRepository::initialize()"
and finally this iterates over a NULLd "$this->overrides", triggering the error.

src/Composer/Repository/PlatformRepository.php

@@ -31,12 +31,11 @@ class PlatformRepository extends ArrayRepository
      *
      * @var array
      */
-    private $overrides;
+    private $overrides = array();
 
     public function __construct(array $packages = array(), array $overrides = array())
     {
         parent::__construct($packages);
-        $this->overrides = array();
         foreach ($overrides as $name => $version) {
             $this->overrides[strtolower($name)] = array('name' => $name, 'version' => $version);
         }