ldb: Avoid undefined pointer arithmetic

ldb: Avoid undefined pointer arithmetic

Computing a pointer that points outside of an array, and not to one past
the last element, is undefined behaviour. To avoid this, do our
comparisons in terms of lengths, not pointers.

Signed-off-by: Joseph Sutton <josephsutton@catalyst.net.nz>
Reviewed-by: Andreas Schneider <asn@samba.org>