Only use "platform" config if it is an array.

Another option could have been to adjust the test that failed so a
platform array is returned from the mock config object but this
feels more robust.

If "platform" config is set to a non-array value it is silently
ignored now.

src/Composer/Installer.php

@@ -196,7 +196,9 @@ class Installer
 
         // create installed repo, this contains all local packages + platform packages (php & extensions)
         $localRepo = $this->repositoryManager->getLocalRepository();
-        $platformRepo = new PlatformRepository($this->config->get('platform'));
+        $platformOverride = $this->config->get('platform');
+        $platformOverride = is_array($platformOverride) ? $platformOverride : array();
+        $platformRepo = new PlatformRepository($platformOverride);
         $repos = array(
             $localRepo,
             new InstalledArrayRepository(array($installedRootPackage)),