Fix disconnection from pointer to member signal that belongs to the base
class, but whose type is a pointer to a member of the derived class.
Commit 9cc106d9d7 fixed connect, so apply
the same fix in disconnect
[ChangeLog][QtCore][QObject] Fixed disconnecting from pointer to member
signal that belongs in the base class but whose type is explicitly given
as a pointer to a member in the derived class
Task-number: QTBUG-40638
Change-Id: Ia546fc8f36e1ea0dd0645bdd820aea47f43677ac
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>