drupal_check_profile should check data retrieve from hook_requirements

Hi,

No checks are done on data retrieve from hook_requirements callback called in drupal_check_profile(). Not like other functions such as system_status() and update_check_requirements() which do it implicitly by using module_invoke_all().

As a consequence, it could try to merge an array with null if function callback doesn't return anything (which should throw php warning). Finally, requirements array would be null. if install_check_requirements() does nothing on this variable, it still be null and the merge done in install_verify_requirements()

$requirements += drupal_verify_profile($install_state);

will generate an unsupported operand types PHP error.