Expelled even the innocent zeroing of destructed object references from

the gc check pass. It's upsetting the debug checks, and it's cleaner to
do all zapping of refs in the mark/cycle check pass anyway.

Rev: src/array.c:1.74
Rev: src/gc.c:1.101
Rev: src/gc.h:1.53
Rev: src/mapping.c:1.90
Rev: src/object.c:1.131
Rev: src/svalue.c:1.81
Rev: src/svalue.h:1.60