s3:registry: fix the test for a REG_SZ blob possibly being a zero terminated ucs2...

s3:registry: fix the test for a REG_SZ blob possibly being a zero terminated ucs2 string

1. catch data blobs with odd number of bytes (not an ucs2 string at all)
2. test the right ucs2 character to be 0
   (prevent out-of bounds access/potential segfault)

Autobuild-User: Michael Adam <obnox@samba.org>
Autobuild-Date: Sun Oct  2 01:26:05 CEST 2011 on sn-devel-104
(cherry picked from commit 95bb2c23e6e9c52a1e34916dff05b1d306278bc6)

The last 2 patches address bug #8528 (SEGFAULT from net registry export on not
zero terminated REG_SZ values).