tdf#148143: Reset the position hint when flat_segment_tree gets copied

std::optional stores the wrapped object as part of its memory footprint,
and when it gets copied, it copy-constructs the wrapped object too.  We
need to be aware of this when using std::optional inside std::vector
which may reallocate its internal buffer and copy the stored elements.

Change-Id: Ib8fab1295388ae2ee9ef3d044943ac3c2bc8e529
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/148152
Tested-by: Jenkins
Reviewed-by: Kohei Yoshida <kohei@libreoffice.org>

sc/inc/columnspanset.hxx

@@ -58,6 +58,7 @@ private:
         ColumnSpansType::const_iterator miPos;
 
         ColumnType(SCROW nStart, SCROW nEnd, bool bInit);
+        ColumnType(const ColumnType& rOther);
     };
 
     typedef std::vector<std::optional<ColumnType>> TableType;

sc/source/core/data/columnspanset.cxx

@@ -53,6 +53,9 @@ ColRowSpan::ColRowSpan(SCCOLROW nStart, SCCOLROW nEnd) : mnStart(nStart), mnEnd(
 ColumnSpanSet::ColumnType::ColumnType(SCROW nStart, SCROW nEnd, bool bInit) :
     maSpans(nStart, nEnd+1, bInit), miPos(maSpans.begin()) {}
 
+ColumnSpanSet::ColumnType::ColumnType(const ColumnType& rOther) :
+    maSpans(rOther.maSpans), miPos(maSpans.begin()) {} // NB: copying maSpans invalidates miPos - reset it
+
 ColumnSpanSet::Action::~Action() {}
 void ColumnSpanSet::Action::startColumn(SCTAB /*nTab*/, SCCOL /*nCol*/) {}
 
@@ -142,6 +145,7 @@ void ColumnSpanSet::scan(
 
         ColumnNonEmptyRangesScanner aScanner(rCol.maSpans, bVal);
         ParseBlock(rSrcCells.begin(), rSrcCells, aScanner, nRow1, nRow2);
+        rCol.miPos = rCol.maSpans.begin();
     }
 }