Don't deadlock when deleting slot objects in QMetaObject::activate()

The slot object was deleted after the mutex was relocked, which caused
a deadlock in case the functor destructor locked the same mutex again.

Change-Id: I5b4fb22fdb4483f91c89915872bfd548c31b0eea
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>