This is a small patch for #6198 (closed), to memwipe a little more. It's a defense-in-depth thing. I'm marking it for possible backport, but I'm not sure whether that's a good idea.
Github PR to master at https://github.com/torproject/tor/pull/2043
After merging this, we could open a new ticket about refactoring more places in our code to use
tor_str_wipe_and_free(), but IMO such a change should be master-only.