librpc/ndr/uuid.c: improve speed and accuracy of GUID string parsing

librpc/ndr/uuid.c: improve speed and accuracy of GUID string parsing

GUID_from_data_blob() was relying on sscanf to parse strings, which was
slow and quite accepting of invalid GUIDs. Instead we directly read a
fixed number of hex bytes for each field.

This now passes the samba4.local.ndr.*.guid_from_string_invalid tests.

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Autobuild-User(master): Douglas Bagnall <dbagnall@samba.org>
Autobuild-Date(master): Wed Dec 14 08:55:42 CET 2016 on sn-devel-144