crypto.c: zero out sensitive data before freeing it

1. crypto_pk_read_private_key_from_filename(): tor_free(contents)
2. crypto_pk_write_private_key_to_filename(): tor_free(s)
3. crypto_dh_compute_secret(): tor_free(secret_tmp)

added component::core tor/tor priority::medium resolution::fixed status::closed tor-relay type::defect labels

Trac:
Milestone: N/A to Tor: 0.2.1.x-final

Fixed in branch bug2384, I hope.

Trac:
Status: new to needs_review

a42adfbf43025d looks fine.

I have not audited to see if we're missing anything, but what we did shouldn't (ha) break anything.

merged

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

Trac:
Keywords: N/A deleted, tor-relay added

Trac:
Component: Tor Relay to Tor

Milestone Tor: 0.2.1.x-final deleted

Trac:
Milestone: Tor: 0.2.1.x-final to N/A

closed

moved to tpo/core/tor#2384 (closed)