The problem is that Modify() may be called from ~SwFmt while another
thread is waiting on SolarMutex in the ~SwXFrame or derived class
destructor, so the ref-count is 0 and the uno::Reference in Modify()
will cause a double-free.

Since ClientModify() does some re-parenting of listeners, it is at first
glance better to call Modify() in this situation (although it is quite
possible that it actually doesn't matter), so don't avoid the call in
~SwFrmFmt by adding a SwClient* member there, but put a weak reference
to itself into SwXFrame so it can check if it's still alive...

Change-Id: I492bb8a8557af5fc725fdb7f8b21013e0886f63b