Add assertions about held lock when accessing all_bottom_indices

Add assertions about held lock when accessing all_bottom_indices
(code refactoring)

* headers.c (GC_all_bottom_indices, GC_all_bottom_indices_end): Add
comment that the variable should be accessed with the allocation lock
held.
* headers.c (get_index, GC_apply_to_all_blocks, GC_next_used_block,
GC_prev_block): Add assertion (at the beginning of the function) that
the allocation lock is held.