libweston: Support NULL weston_pointer in init_pointer_constraint

Fix init_pointer_constraint so that it creates a valid, but inert,
resource if a NULL weston_pointer value is passed in. In that case no
constraint object is associated with the resource, but this is not an
issue since affected code can already handle NULL constraint objects.

Signed-off-by: Alexandros Frantzis <alexandros.frantzis@collabora.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>

libweston/input.c

@@ -3572,7 +3572,7 @@ init_pointer_constraint(struct wl_resource *pointer_constraints_resource,
 	struct wl_resource *cr;
 	struct weston_pointer_constraint *constraint;
 
-	if (get_pointer_constraint_for_pointer(surface, pointer)) {
+	if (pointer && get_pointer_constraint_for_pointer(surface, pointer)) {
 		wl_resource_post_error(pointer_constraints_resource,
 				       ZWP_POINTER_CONSTRAINTS_V1_ERROR_ALREADY_CONSTRAINED,
 				       "the pointer has a lock/confine request on this surface");
@@ -3587,6 +3587,7 @@ init_pointer_constraint(struct wl_resource *pointer_constraints_resource,
 		return;
 	}
 
+	if (pointer) {
 		constraint = weston_pointer_constraint_create(surface, pointer,
 							      region, lifetime,
 							      cr, grab_interface);
@@ -3594,10 +3595,14 @@ init_pointer_constraint(struct wl_resource *pointer_constraints_resource,
 			wl_client_post_no_memory(client);
 			return;
 		}
+	} else {
+		constraint = NULL;
+	}
 
 	wl_resource_set_implementation(cr, implementation, constraint,
 				       pointer_constraint_constrain_resource_destroyed);
 
+	if (constraint)
 		maybe_enable_pointer_constraint(constraint);
 }