tdb: make check more robust against recovery failures.

tdb: make check more robust against recovery failures.

We can end up with dead areas when we die during transaction commit;
tdb_check() fails on such a (valid) database.

This is particularly noticable now we no longer truncate on recovery;
if the recovery area was at the end of the file we used to remove it
that way.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>