Add nonfatal_once assertion about is_canonical assertion in `circuit_deliver_create_cell()`

Working on #13753 (closed): it seems the best way to do that is to add a check for canonicity in circuit_deliver_create_cell().

changed milestone to %Tor: 0.4.5.x-freeze

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mentioned in issue #13753 (closed)

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changed title from Add nonfatal_once about is_canonical assertion in circuit_deliver_create_cell() to Add nonfatal_once assertion about is_canonical assertion in circuit_deliver_create_cell()

changed time estimate to 15m

This is trivial, but it has to happen after #40080 (closed) and #40081 (closed)

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changed milestone to %Tor: 0.4.5.x-stable

This happened as part of 435f31ae

closed