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).

rts/include/rts/storage/Block.h

@@ -112,6 +112,11 @@ typedef struct bdescr_ {
                                    // value (StgPtr)(-1) is used to
                                    // indicate that a block is unallocated.
                                    //
+                                   // Also note that this field is only set lazily
+                                   // (e.g. when the mutator starts and stops
+                                   // allocating in the nursery stg_gc_noregs) in
+                                   // the case of a nursery block.
+                                   //
                                    // Unused by the non-moving allocator.
         struct NonmovingSegmentInfo nonmoving_segment;
     };