clipboard: zalloc the clipboard_source

The wrapped weston_data_source struct has new fields which were left
uninitialized, so its access is unreliable.

Signed-off-by: Carlos Garnacho <carlosg@gnome.org>
Reviewed-by: Derek Foreman <derekf@osg.samsung.com
Tested-by: Derek Foreman <derekf@osg.samsung.com>

src/clipboard.c

@@ -141,7 +141,7 @@ clipboard_source_create(struct clipboard *clipboard,
 	struct clipboard_source *source;
 	char **s;
 
-	source = malloc(sizeof *source);
+	source = zalloc(sizeof *source);
 	if (source == NULL)
 		return NULL;