compositor: Handle OOM in weston_buffer_from_resource()

Handle NULL-return in call-sites as well.
diff --git a/src/screenshooter.c b/src/screenshooter.c
index 2414015..a89194e 100644
--- a/src/screenshooter.c
+++ b/src/screenshooter.c
@@ -180,6 +180,10 @@
 	struct weston_buffer *buffer =
 		weston_buffer_from_resource(buffer_resource);
 
+	if (buffer == NULL) {
+		wl_resource_post_no_memory(resource);
+		return;
+	}
 	if (!wl_shm_buffer_get(buffer->resource))
 		return;