Draft: rts: Clear bd->free in the DEBUG RTS
In the non-DEBUG RTS we initialize bd->free
lazily (e.g. when
the mutator starts allocating into the block in stg_gc_noregs).
However, in the past we have had bugs where code looked at
the free
field of blocks that the mutator never allocated into.
We set the free pointer to NULL to catch this.
This would help to catch #16862 (closed).
Edited by Ben Gamari