Fixed $wgCookieExpiration functionality when set to 0.

When $wgCookieExpiration is set to 0, cookies should
by default expire when the browser closes. However,
MediaWiki accidentally interpreted this as the cookies
expiring 0 seconds from the request time.

Bug: 47886
Change-Id: Ib988ad18574122a56b0d11c8888c7c41d94dea6e