Fix crash on edit s, p, u key 'U' with negative argument

Fix crash on edit s, p, u key 'U' with negative argument

ef_ensure_space() oopses on negative ID, but succeeds anyway.  edit()
proceeds to ef_write(), which neglects to check for negative ID.
Since the ID isn't in the cache, it then passes a NULL old element to
callback prewrite(), which crashes.

Fix ef_ensure_space() to fail on negative ID.  Commit 5173f8cd
(v4.3.0) made it oops, but neglected to make it fail.

Fix ef_write() to oops and fail on negative ID.

ef_write() still passes NULL old element to prewrite() when the ID
isn't in the cache.  Doesn't actually happen, because we use
prewrite() callbacks only with fully cached tables.  Fragile.  Make
ef_open() fail when that assumption is violated.