add wl_abort private function

On many places in the code we use wl_log + abort or wl_log + assert(0).
Replace these with one call to wl_abort, so that we don't mix abort(),
assert(0) and we'll save few lines

Signed-off-by: Marek Chalupa <mchqwerty@gmail.com>
Reviewed-by: Derek Foreman <derekf@osg.samsung.com>