Commit 8a6787bd54 added screen.fake_add(). Commit 08845c7a4b made us cache a
screen's workarea in the struct screen_t. This new member needs to be
initialized to the screen's geometry when a new screen is added. Since both
these commits were developed concurrently, the workarea was not initialized in
screen.fake_add().
Fix this by calling in fake_add() the helper function added in 08845c7a4b.
Signed-off-by: Uli Schlachter <psychon@znc.in>