Both explicit compaction routines (gc_compact and the verify references form)
need to clear the heap before executing compaction.  Otherwise some
objects may not be alive, and we'll need the read barrier.  The heap
must only contain *live* objects if we want to disable the read barrier
during explicit compaction.

The previous commit was missing the "clear the heap" phase from the
"verify references" explicit compaction function.

Fixes [Bug #17306]