tor_remove_file() should ignore NULL filenames

Otherwise, tor logs warnings on my macOS box that look like:

Couldn't unlink (null): Bad address

changed milestone to %Tor: 0.3.3.x-final in legacy/trac

added actualpoints::0.1 in Legacy / Trac component::core tor/tor in Legacy / Trac milestone::Tor: 0.3.3.x-final in Legacy / Trac owner::teor in Legacy / Trac points::0.1 in Legacy / Trac priority::medium in Legacy / Trac resolution::fixed in Legacy / Trac severity::normal in Legacy / Trac status::closed in Legacy / Trac tor-shutdown in Legacy / Trac type::defect in Legacy / Trac labels

Please see my branch bug24762 on https://github.com/teor2345/tor.git

Trac:
Status: assigned to needs_review

Trac:
Actualpoints: N/A to 0.1

lgtm; merged to master

Trac:
Status: needs_review to closed
Resolution: N/A to fixed

closed

changed time estimate to 48m

added 48m of time spent

moved from legacy/trac#24762 (moved)

added Bug label and removed 1 deleted label

removed 1 deleted label