s4-dsdb guard principalName parse for invalid inputs

s4-dsdb guard principalName parse for invalid inputs

We need to ensure that if this parses name.name_string as just one
val, then we don't read uninitialised and possibly unallocated memory.
Found by Adam Thorn <alt36@cam.ac.uk>

While we are checking that, we need to fix the strncasecmp() check to
first check if the string is the expected length, then check for a
match against sAMAccountName-without-doller, as otherwise we will
permit a string such as machinefoo to match a sAMAccountName of
machine.

Andrew Bartlett

Autobuild-User: Andrew Bartlett <abartlet@samba.org>
Autobuild-Date: Fri Jul  1 03:55:00 CEST 2011 on sn-devel-104