Fix unsafe_write_addr_at to convert word to byte offsets
Fix unsafe_write_addr_at to convert word to byte offsets

Summary:
In the heap, we store address as byte offsets. In OCaml, we represent addresses
as word offsets, for use as indices into the word-sized Bigarray.

In D24393641 (https://github.com/facebook/flow/commit/ea749eb8ad69909aead58b8b043d3e8120595af0), I ensured that word<->byte offset conversions happen over the
OCaml/C boundary, but I neglected to handle this case, which writes directly
from OCaml into the heap.

Reviewed By: nmote

Differential Revision: D24595231

fbshipit-source-id: a0049a6dc0065389cbd9fb7e0926c2aea0caf5b4

mroch/flow,mroch/flow,mroch/flow,mroch/flow,facebook/flow,facebook/flow,facebook/flow,facebook/flow,facebook/flow,mroch/flow,mroch/flow,facebook/flow,mroch/flow,facebook/flow