Fix crash when using QLockFile in a global destructor

(for instance any global object which writes out to a config file in the
destructor). If the global cache isn't available anymore, don't use it.

Change-Id: I851a6e394d0b073aebf3ffd88b1966d424bfb92e
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>