clipboard: pre-condition on selection value <= 255

The protocol uses a u8 for the selection value. Make sure the given
argument value fits there, or throw a critical.

The other places seem to use u8 variables already.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>

src/channel-main.c

@@ -1354,6 +1354,7 @@ static void agent_clipboard_grab(SpiceMainChannel *channel, guint selection,
     if (!c->agent_connected)
         return;
 
+    g_return_if_fail(selection <= G_MAXUINT8);
     g_return_if_fail(test_agent_cap(channel, VD_AGENT_CAP_CLIPBOARD_BY_DEMAND));
 
     size = sizeof(VDAgentClipboardGrab) + sizeof(uint32_t) * ntypes;