tdb_traverse/tdb_traverse_read: check returns for negative, not -1.

tdb_traverse/tdb_traverse_read: check returns for negative, not -1.

TDB2 returns a negative error number on failure.  This is compatible
if we always check for < 0 instead of == -1.

Also, there's no tdb_traverse_read in TDB2: we don't try to make
traverse reliable any more, so there are no write locks anyway.

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