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Prior to 49b306ecb9
the `optional_arg` passed from `rb_hash_update_block_i` to `tbl_update`
was a hash value (i.e. a VALUE). After that commit it changed to an
`update_call_args`.
If the block sets or changes the value, `tbl_update_modify` will set the
`arg.value` back to an actual value and we won't crash. But in the case
where the block returns the original value we end up calling
`RB_OBJ_WRITTEN` with the `update_call_args` which is not expected and
may crash.
`arg.value` appears to only be used to pass to `RB_OBJ_WRITTEN` (others
who need the `update_call_args` get it from `arg.arg`), so I don't think
it needs to be set to anything upfront. And `tbl_update_modify` will set
the `arg.value` in the cases we need the write barrier.