* libguile/smob.c (clear_smobnum): New helper.
(finalize_smob): Re-set the smobnum to the "finalized smob" type
before finalizing. Fixes#19883.
(scm_smob_prehistory): Pre-register a "finalized smob" type, which has
no mark procedure.
* test-suite/standalone/test-smob-mark-race.c: New file.
* test-suite/standalone/Makefile.am: Arrange to build and run the new
test.
