libutils: wp::fromExisting bugfix

This API was tested before, but it wasn't used until it is needed in
libbinder. Previously it passed the tests because wp::operator==
compares weak_ptrs which are never deleted, but it doesn't check the
value of m_ptr as well. This assumes that the RefBase implementation is
self-consistent.

Future considerations:
- add additional CHECK (perf?)
- add an additional optional CHECK?
- update all refbase tests to use an embellished form of this operator

Bug: 184190315
Test: libutils_test, boot and kill process when libbinder is using this
API

Change-Id: I66c97386d769529d5efae48e06775d4b4a344025