tdb2: be more careful on 4G files (tdb1).

I came across a tdb which had wrapped to 4G + 4K, and the contents had been
destroyed by processes which thought it only 4k long.  Fix this by checking
on open, and making tdb_oob() check for wrap itself.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
(Ported from tdb1 SAMBA commit b64494535dc62f4073fc6302847593ed6e6ec38b)