When looking up an entry in the rend_cache, stop asserting that it exists but
rather confirm it exists and if not, return that no entry was found.

The reason for that is because the hs_circ_cleanup_on_free() function (which
can end up looking at the rend_cache) can be called from the
circuit_free_all() function that is called _after_ the rend cache is cleaned
up in tor_free_all().

We could fix the free all ordering but then it will just hide a future bug.
Instead, handle a missing rend_cache as a valid use case as in while we are in
the cleanup process.

As Tor becomes more modular, it is getting more and more difficult to ensure
subsystem callstack ordering thus this fix aims at making the HSv2 subsystem
more robust at being called while tor is pretty much in any kind of state.

Fixes #32847.

Signed-off-by: David Goulet <dgoulet@torproject.org>