QmlDesigner: Fixing double free

Calling detach() in setKey() will free the shared memory twice, if
we do not set m_memory to nullptr.
Clean cleanHandleInternal() does always return true, so
checking the value does not make sense.
We have to call cleanHandleInternal() immediately after the close.

Change-Id: I110693d1dd9dae4ff5e52cfd3fdd2f33137af969
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>