clients: use xmalloc in more places

For the clients continue to use xmalloc() to simplify OOM-handling.

Signed-off-by: Brian Lovin <brian.j.lovin@intel.com>
diff --git a/clients/screenshot.c b/clients/screenshot.c
index 7511ef7..d58d8b1 100644
--- a/clients/screenshot.c
+++ b/clients/screenshot.c
@@ -115,7 +115,7 @@
 	static struct screenshooter_output *output;
 
 	if (strcmp(interface, "wl_output") == 0) {
-		output = malloc(sizeof *output);
+		output = xmalloc(sizeof *output);
 		output->output = wl_registry_bind(registry, name,
 						  &wl_output_interface, 1);
 		wl_list_insert(&output_list, &output->link);