clients: Don't crash when compositor doesn't support drag and drop

display_create_data_source() can return NULL when there's no data device
manager present.  Instead of carrying on blindly, test its return value.

Signed-off-by: Derek Foreman <derekf@osg.samsung.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>

clients/editor.c

@@ -639,6 +639,9 @@ editor_copy_cut(struct editor *editor, struct input *input, bool cut)
 
 		editor->selection =
 			display_create_data_source(editor->display);
+		if (!editor->selection)
+			return;
+
 		wl_data_source_offer(editor->selection,
 				     "text/plain;charset=utf-8");
 		wl_data_source_add_listener(editor->selection,

clients/terminal.c

@@ -2264,6 +2264,9 @@ terminal_copy(struct terminal *terminal, struct input *input)
 {
 	terminal->selection =
 		display_create_data_source(terminal->display);
+	if (!terminal->selection)
+		return;
+
 	wl_data_source_offer(terminal->selection,
 			     "text/plain;charset=utf-8");
 	wl_data_source_add_listener(terminal->selection,