fix bug in ScTable::LastHiddenColRow

found by inspection, noticed that the for loop is not actually using
it's index variable

introduced in

    commit b3579d71
    koheirowlimitperf: #i109369# #i109373# #i109384# #i109385# #i109386#
million, and integrated lots of speed optimization and bug fixes to
ensure Calc remains usable after the row limit increase.

Change-Id: Icfee19ddb051e8d4333ed4c91d384379bf494444
Reviewed-on: https://gerrit.libreoffice.org/41439Reviewed-by: Julien Nabet <serval2412@yahoo.fr>
Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>

sc/source/core/data/table5.cxx

@@ -764,8 +764,8 @@ SCCOLROW ScTable::LastHiddenColRow(SCCOLROW nPos, bool bCol) const
         {
             for (SCCOL i = nCol+1; i <= MAXCOL; ++i)
             {
-                if (!ColHidden(nCol))
-                    return nCol - 1;
+                if (!ColHidden(i))
+                    return i - 1;
             }
         }
     }