fix assert seen on opening attachment from tdf#123163

I imagine it should have been seqPos-(idPos+2)

seems to be like this since the initial commit of

commit 091fe76b
Date:   Tue Jun 30 12:55:18 2015 +0300

    tdf#87348 implement mso-next-textbox vml-style textbox chaining import

Change-Id: Ic2f527ede2102c01c8589d58d8c705d59b0a6ffe
Reviewed-on: https://gerrit.libreoffice.org/67450Reviewed-by: Justin Luth <justin_luth@sil.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

oox/source/vml/vmlshape.cxx

@@ -367,7 +367,8 @@ Reference< XShape > ShapeBase::convertAndInsert( const Reference< XShapes >& rxS
                     sal_Int32 seqPos = sLinkChainName.indexOf("_s",idPos);
                     if (idPos < seqPos)
                     {
-                        id = sLinkChainName.copy(idPos+2,seqPos-idPos+2).toInt32();
+                        auto idPosEnd = idPos+2;
+                        id = sLinkChainName.copy(idPosEnd, seqPos - idPosEnd).toInt32();
                         seq = sLinkChainName.copy(seqPos+2).toInt32();
                     }
                 }