Initialize result->backing_object_offset before linking result onto the list of

Initialize result->backing_object_offset before linking result onto the list of
vm objects shadowing source in vm_object_shadow().  This closes a race where
vm_object_collapse() could be called with a partially uninitialized object
argument causing symptoms that looked like hardware problems, e.g.  signal 6,
10, 11 or a /bin/sh busy-waiting for a nonexistant child process.