Check whether canary class fits
Check whether canary class fits

Summary: Use the regular insertion for the canary classes, and add an assert that the insertion did not fail. This will alert earlier to overflow issues and help diagnosis of later breaks.

Reviewed By: Feng23

Differential Revision: D31926756

fbshipit-source-id: 7ceeeaae572d9619695dcf4c603b0963b513fc2c

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