ofz#8252 fallout from rework as OUStringBuffer

Change-Id: Ide3732627eb145e191061608ac0325df0fcceff0
Reviewed-on: https://gerrit.libreoffice.org/54198Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

starmath/source/mathtype.cxx

@@ -2646,7 +2646,8 @@ bool MathType::HandlePile(int &rSetAlign, int nLevel, sal_uInt8 nSelector, sal_u
 
     rRet.append(" stack {\n");
     bool bRet = HandleRecords( nLevel+1, nSelector, nVariation, -1, -1 );
-    rRet.remove(rRet.getLength()-3, 2);
+    int nRemoveFrom = rRet.getLength() >= 3 ? rRet.getLength() - 3 : 0;
+    rRet.remove(nRemoveFrom, 2);
     rRet.append("} ");
 
     while (rSetAlign)