Added a check for the depot repoConfig in the PerforceDriver

After the last composer self-update i get the following Exception. I don't use Perforce, maybe this is a valid Patch. I wasn't able to test it

Cheers
Sven
                          
  [ErrorException]        
  Undefined index: depot  
                          


Exception trace:
 () at phar:///composer.phar/src/Composer/Repository/Vcs/PerforceDriver.php:35
 Composer\Util\ErrorHandler::handle() at phar:///composer.phar/src/Composer/Repository/Vcs/PerforceDriver.php:35
 Composer\Repository\Vcs\PerforceDriver->initialize() at phar:///composer.phar/src/Composer/Repository/VcsRepository.php:85
 Composer\Repository\VcsRepository->getDriver() at phar:///composer.phar/src/Composer/Repository/VcsRepository.php:112
 Composer\Repository\VcsRepository->initialize() at phar:///composer.phar/src/Composer/Repository/ArrayRepository.php:170
 Composer\Repository\ArrayRepository->getPackages() at phar:///composer.phar/src/Composer/DependencyResolver/Pool.php:158
 Composer\DependencyResolver\Pool->addRepository() at phar:///composer.phar/src/Composer/Installer.php:337
 Composer\Installer->doInstall() at phar:///composer.phar/src/Composer/Installer.php:211
 Composer\Installer->run() at phar:///composer.phar/src/Composer/Command/UpdateCommand.php:116
 Composer\Command\UpdateCommand->execute() at phar:///composer.phar/vendor/symfony/console/Symfony/Component/Console/Command/Command.php:244
 Symfony\Component\Console\Command\Command->run() at phar:///composer.phar/vendor/symfony/console/Symfony/Component/Console/Application.php:897
 Symfony\Component\Console\Application->doRunCommand() at phar:///composer.phar/vendor/symfony/console/Symfony/Component/Console/Application.php:191
 Symfony\Component\Console\Application->doRun() at phar:///composer.phar/src/Composer/Console/Application.php:117
 Composer\Console\Application->doRun() at phar:///composer.phar/vendor/symfony/console/Symfony/Component/Console/Application.php:121
 Symfony\Component\Console\Application->run() at phar:///composer.phar/src/Composer/Console/Application.php:83
 Composer\Console\Application->run() at phar:///composer.phar/bin/composer:43
 require() at /composer.phar:15