simplify expression

Change-Id: I57c7df50040737656c2a68e5fdc0d069ad21f833
Reviewed-on: https://gerrit.libreoffice.org/61975
Tested-by: Jenkins
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

sot/source/sdstor/stgio.cxx

@@ -211,8 +211,9 @@ FatError EasyFat::Mark( sal_Int32 nPage, sal_Int32 nCount, sal_Int32 nExpect )
 {
     if( nCount > 0 )
     {
-        --nCount /= GetPageSize();
-        nCount++;
+        --nCount;
+        nCount /= GetPageSize();
+        ++nCount;
     }
 
     sal_Int32 nCurPage = nPage;