Make LLInstanceTracker<T, T*>::getInstance(T*) validate passed T*.

For the T* specialization (no string, or whatever, key), the original
getInstance() method simply returned the passed-in T* value. It was defined,
as the comments noted, for completeness of the analogy with the keyed
LLInstanceTracker specialization.
It turns out, though, that getInstance(T*) can still be useful to ask whether
the T* you have in hand still references a valid T instance. Support that
usage.