Check zalloc return for out of memory situation

Most zalloc calls in weston are checked, this fixes a handful that were
being ignored.  As found by `grep -EIsr "[^x]zalloc\(" . -A1`

Signed-off-by: Bryce Harrington <b.harrington@samsung.com>
diff --git a/src/libinput-seat.c b/src/libinput-seat.c
index 8bf538c..acb29d7 100644
--- a/src/libinput-seat.c
+++ b/src/libinput-seat.c
@@ -330,9 +330,9 @@
 	struct udev_seat *seat;
 
 	seat = zalloc(sizeof *seat);
-
 	if (!seat)
 		return NULL;
+
 	weston_seat_init(&seat->base, c, seat_name);
 	seat->base.led_update = udev_seat_led_update;