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>

src/wayland-private.h

@@ -209,6 +209,7 @@ wl_closure_destroy(struct wl_closure *closure);
 extern wl_log_func_t wl_log_handler;
 
 void wl_log(const char *fmt, ...);
+void wl_abort(const char *fmt, ...);
 
 struct wl_display;