(ef_read): Catch null cache.

(ef_read): Catch null cache.

(ef_read): Used to get the file size again some circumstances; doesn't
make sense, as no other process may be writing this file.

(ef_write): Don't refuse to write beyond record 65536.  This module
can cope with that.  Some users have trouble long before that (short
record numbers), but that should be handled there.

(ef_write): Catch table not file-backed.