Fix resource path check when ResourceBasePath is an empty string

Fix resource path check when ResourceBasePath is an empty string

If you have MediaWiki installed in the root of the domain, then
$wgScriptPath and $wgResourceBasePath is an empty string. In HHVM and
PHP, passing an empty string as the second parameter of strpos() causes
a warning and returns false, which will cause the condition to fail, as
if the path were not within the base path.

So, normalize such paths. Using substr() instead of strpos() for a
"starts with" check would have worked except that RelPath also fails
when given an empty string.

Bug: T127652
Change-Id: If7e94ae638d6834f7cc0f31f67a5fe6a2f74771c