input: Use slightly less obscure logic in evdev_notify_keyboard_focus()

While the test is actually correct (for non-negative numbers), it's not
at all clear and seems to be an accidental order of operations mistake.

Also, add an assert() to make sure this number is never negative.

Closes bug 86346 - https://bugs.freedesktop.org/show_bug.cgi?id=86346

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

src/input.c

@@ -2168,6 +2168,7 @@ WL_EXPORT void
 weston_seat_release_keyboard(struct weston_seat *seat)
 {
 	seat->keyboard_device_count--;
+	assert(seat->keyboard_device_count >= 0);
 	if (seat->keyboard_device_count == 0) {
 		weston_keyboard_set_focus(seat->keyboard, NULL);
 		weston_keyboard_cancel_grab(seat->keyboard);

src/libinput-device.c

@@ -470,7 +470,7 @@ evdev_notify_keyboard_focus(struct weston_seat *seat,
 {
 	struct wl_array keys;
 
-	if (!seat->keyboard_device_count > 0)
+	if (seat->keyboard_device_count == 0)
 		return;
 
 	wl_array_init(&keys);