Updater: Avoid fatal on wfLoadSkin/wfLoadExtension

Updater: Avoid fatal on wfLoadSkin/wfLoadExtension

When having $wgExtensionDirectory or $wgStyleDirectory set to a
different location in LocalSettings.php, the load of LocalSettings.php
can give fatal, because the called functions wfLoadSkin and
wfLoadExtensions need at least this globals to be set correctly.

This does not bring the vector skin back to the updater, when having the
globals set, but it avoids a fatal about "MonoBook/skin.json does not
exist!" or similar.

Change-Id: I095c545f77aa50d6be4cd48588bd1ae1c82cf343