ldb_tdb: A more robust check for if we can fit the index string in

ldb_tdb: A more robust check for if we can fit the index string in

This avoids magic numbers and also is careful against overflow
from a long attr_for_dn.

This is done as a distinct commit to make the previous behaviour
change more clear, and to show that this does not change the
calculations, only improves the overflow check.

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Gary Lockyer <gary@catalyst.net.nz>