Fix crash reported by Kornel.

See https://marc.info/?l=lyx-devel&m=155879185229073&w=2.

The problem is that, after saving the document and reloading, the
TOC is corrupted, more or less, when we run through updateBuffer.
So we reset it first.

src/Buffer.cpp

@@ -4980,6 +4980,10 @@ void Buffer::updateBuffer(UpdateScope scope, UpdateType utype) const
 	setChangesPresent(false);
 
 	Buffer & cbuf = const_cast<Buffer &>(*this);
+	// if we are reloading, then we could have a dangling TOC,
+	// in effect. so we need to go ahead and reset, even though
+	// we will do so again when we rebuild the TOC later.
+	cbuf.tocBackend().reset();
 
 	// do the real work
 	ParIterator parit = cbuf.par_iterator_begin();

src/TocBackend.cpp

@@ -204,17 +204,24 @@ bool TocBackend::updateItem(DocIterator const & dit_in)
 }
 
 
-void TocBackend::update(bool output_active, UpdateType utype)
+void TocBackend::reset()
 {
-	for (TocList::iterator it = tocs_.begin(); it != tocs_.end(); ++it)
-		it->second->clear();
+	for (auto const & t: tocs_)
+		t.second->clear();
 	tocs_.clear();
 	builders_.clear();
 	resetOutlinerNames();
-	if (!buffer_->isInternal()) {
-		DocIterator dit;
-		buffer_->inset().addToToc(dit, output_active, utype, *this);
-	}
+}
+
+
+void TocBackend::update(bool output_active, UpdateType utype)
+{
+	reset();
+	if (buffer_->isInternal())
+		return;
+
+	DocIterator dit;
+	buffer_->inset().addToToc(dit, output_active, utype, *this);
 }
 
 

src/TocBackend.h

@@ -113,6 +113,8 @@ public:
 	void setBuffer(Buffer const * buffer) { buffer_ = buffer; }
 	///
 	void update(bool output_active, UpdateType utype);
+	///
+	void reset();
 	/// \return true if the item was updated.
 	bool updateItem(DocIterator const & pit);
 	///