Fix `unused-result' warnings.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
diff --git a/clients/window.c b/clients/window.c
index 73b7e88..9f5b837 100644
--- a/clients/window.c
+++ b/clients/window.c
@@ -1984,7 +1984,9 @@
 {
 	int p[2];
 
-	pipe2(p, O_CLOEXEC);
+	if (pipe2(p, O_CLOEXEC) == -1)
+		return;
+
 	wl_data_offer_receive(offer->offer, mime_type, p[1]);
 	close(p[1]);