Fix ancient code that only checked circ_id, not circ_id and chan

This code mis-handled the case where a circuit got the same circuit
ID in both directions.  I found three instances of it in the
codebase, by grepping for [pn]_circ_id.

Because of the issue in command_process_relay_cell(), this would
have made roughly one circuit in a million completely nonfunctional.

Fixes bug 12195.
Merge remote-tracking branch 'public/bug12195'