generateGUIDString: Fix incorrect string length

This bug caused signature line IDs in OOXML to be written
with an appended "_x0000_".

Change-Id: If219279a5d5177ec5fac2d9dd747dc015c16b6bc
Reviewed-on: https://gerrit.libreoffice.org/60736Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>
Tested-by: Jenkins
Reviewed-by: Samuel Mehrbrodt <Samuel.Mehrbrodt@cib.de>

comphelper/source/xml/xmltools.cxx

@@ -102,7 +102,7 @@ namespace comphelper
                     aSeq[0], aSeq[1], aSeq[2], aSeq[3], aSeq[4], aSeq[5], aSeq[6], aSeq[7], aSeq[8],
                     aSeq[9], aSeq[10], aSeq[11], aSeq[12], aSeq[13], aSeq[14], aSeq[15]);
 
-            return OString(str, SAL_N_ELEMENTS(str));
+            return OString(str);
         }
     }
 }