ofz#817 nStrLen-1 changed to nStrLen-3

regression from...

commit ff8f6629
Author: Caolán McNamara <caolanm@redhat.com>
Date:   Thu Jan 19 16:56:34 2017 +0000

    Resolves: ofz#424 guard against broken dxary length

but this weird typo doesn't appears in the 5-2 and 5-3 backports,
odd how I managed that

Change-Id: I5fb1db2284d48ee78e717d41274a3d37ab0255cf

vcl/source/gdi/svmconverter.cxx

@@ -988,7 +988,7 @@ void SVMConverter::ImplConvertFromSVM1( SvStream& rIStm, GDIMetaFile& rMtf )
                                     // difference to last elem and store
                                     // in very last.
                                     if( nStrLen > 1 )
-                                        pDXAry[ nStrLen-3 ] = pDXAry[ nStrLen-2 ] + pTmpAry[ nStrLen-1 ] - pTmpAry[ nStrLen-2 ];
+                                        pDXAry[ nStrLen-1 ] = pDXAry[ nStrLen-2 ] + pTmpAry[ nStrLen-1 ] - pTmpAry[ nStrLen-2 ];
                                     else
                                         pDXAry[ nStrLen-1 ] = pTmpAry[ nStrLen-1 ]; // len=1: 0th position taken to be 0
                                 }