Fix a crasher with RefBase debugging and vectors of wp<>

background:
we have some code to fix-up the IDs of references when
using RefBase's DEBUG_REFS when those refs are managed by
arrays wp<> or sp<> (this is because wp<> / sp<> don't have
a trivial ctor when DEBUG_REFS is enabled, and Vector
treats them as trivial for obvious performance reasons)

this is complicated by the fact that we don't want to have
to recompile everything when enabling DEBUG_REFs (i.e.: the
Vector code cannot know wheter it's enabled or not for its
template stuff).

problem:
there was a bug in the fix-up code for wp<> which was trying
to access the weakref_impl from the RefBase* however, this was
moronic since RefBase could have been destroyed if there wasn't
any more strong refs -- and this happned. Instead we need to get
the weakref_impl directly from the wp<>

Change-Id: Ie16e334204205fdbff142acb9faff8479a78450b