Xi: disallow grabbing disabled devices

Grabbing a disabled (pointer) device will lead to a segfault later
in the myriad of places where we look at the device's spriteInfo - which
will be NULL.

As a workaround, disallow grabbing a disabled device by pretending it's
already grabbed. Since the point of a grab is to receive all events by
that device and disabled devices cannot send events, this should be Good
Enough.

Tested-by: Olivier Fourdan <ofourdan@redhat.com>
Part-of: <https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/1856>

Xi/xigrabdev.c

@@ -81,6 +81,9 @@ ProcXIGrabDevice(ClientPtr client)
     if (ret != Success)
         return ret;
 
+    if (!dev->enabled)
+        return AlreadyGrabbed;
+
     if (!IsMaster(dev))
         stuff->paired_device_mode = GrabModeAsync;