ldb:ldb_msg.c - make "ldb_msg_find_attr_as_*" more robust against invalid values

ldb:ldb_msg.c - make "ldb_msg_find_attr_as_*" more robust against invalid values

- Integer handling was modeled after validate code from "schema_syntax.c".
- Double handling was modeled similar, but with a dynamic buffer.
  I don't know if there is a maximum literal length for double values but an
  allocation shouldn't a problem here since doubles are rare.
- String handlind is enhanced with a terminating "0" test for safety.

Reviewed-by: abartlet + metze
Autobuild-User: Matthias Dieter Wallnöfer <mdw@samba.org>
Autobuild-Date: Thu Apr  7 16:38:57 CEST 2011 on sn-devel-104