Ruby

Make env_clone compaction safe

The original order of events is:

1. Allocate new_body.
2. Peform memcpy into new_body.
3. Create new_env using new_body.

However, if GC compaction runs during step 3, then new_env would not
have yet been created and objects on new_body could move but it would
not be reference updated.

This commit changes the order of the last two events.