dix: Do not send events in keyboard/pointer grab if old and new window are the same.

This fixes a bug in Citrix Workspace App when TransparentKeyPassthrough=Remote is set in wfclient.ini and the session is in window mode. I assume Citrix Workspace App sends a GrabKeyboard although it already has the keyboard grabbed and in it's focus out/in events ungrabs/grabs the keyboard again. This leads to an endless loop where the Citrix Workspace App toggles grab/ungrab ob keyboard and this makes the xserver unresponsive and cpu usage on one core goes to 100%

The fix from 364d6498 did not work here because grabinfo->grab was NULL