YJIT: Squash canary before falling back

Recent flaky canary-related CI failures have all happened while trying
to fall back. It's unclear what is leaving the canary on the stack and
causing gen_send_dynamic() to falsely assume that it should be leaf,
and this patch isn't going to help us find the source. One source I
found is Array#<< with a frozen array, but it's unclear if that's what's
causing the CI failures. I'm somewhat afraid to add a canary check to
rb_longjmp() since that might introduce more flaky failures, and maybe
ones unrelated to YJIT.

See: https://github.com/ruby/ruby/actions/runs/8083502532/job/22086714152
See: https://github.com/ruby/ruby/actions/runs/8066858522/job/22035963315

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