Fix a bug in sp<> and wp<> which could cause memory corruptions

when assigning a smart pointer to another one, we need to make
sure to read all the data we need from the right-hand-side
reference (the assignee) before we decRef the assigned.

This bug would cause linked-list of smart-pointers to fail
miserably.

Change-Id: Ibb554c15fddf909f7737c632b7c80322e80ea93f