clients: Use xmalloc in a few more places

Just changes some places where a malloc failure is unhandled
to our xmalloc function that exit()s a little more gracefully.

Signed-off-by: Derek Foreman <derekf@osg.samsung.com>
Reviewed-by: Bryce Harrington <b.harrington@samsung.com>
diff --git a/clients/window.c b/clients/window.c
index 5099004..9171e8f 100644
--- a/clients/window.c
+++ b/clients/window.c
@@ -5391,7 +5391,7 @@
 	void *data;
 
 	len = cairo_format_stride_for_width(CAIRO_FORMAT_ARGB32, 1);
-	data = malloc(len);
+	data = xmalloc(len);
 	display->dummy_surface =
 		cairo_image_surface_create_for_data(data, CAIRO_FORMAT_ARGB32,
 						    1, 1, len);