Avoid crash when selecting long text

In some (not yet understood) situations, the paragraph metrics cache
is empty in generateSyntheticMouseEvent(). We just avoid a crash in
this case, but there is probably an underlying problem that deserves
being fixed.

Fixes bug #10324.

(cherry picked from commit 4efb129ccb)

src/TextMetrics.h

@@ -94,6 +94,8 @@ public:
 	bool redoParagraph(pit_type const pit);
 	/// Clear cache of paragraph metrics
 	void clear() { par_metrics_.clear(); }
+	/// Is cache of paragraph metrics empty ?
+	bool empty() const { return par_metrics_.empty(); }
 
 	///
 	int ascent() const { return dim_.asc; }

src/frontends/qt4/GuiWorkArea.cpp

@@ -1009,12 +1009,18 @@ void GuiWorkArea::generateSyntheticMouseEvent()
 
 	// In which paragraph do we have to set the cursor ?
 	Cursor & cur = d->buffer_view_->cursor();
-	// FIXME: we don't know howto handle math.
+	// FIXME: we don't know how to handle math.
 	Text * text = cur.text();
 	if (!text)
 		return;
 	TextMetrics const & tm = d->buffer_view_->textMetrics(text);
 
+	// Quit gracefully if there are no metrics, since otherwise next
+	// line would crash (bug #10324).
+	// This situation seems related to a (not yet understood) timing problem.
+	if (tm.empty())
+		return;
+
 	pair<pit_type, const ParagraphMetrics *> pp = up ? tm.first() : tm.last();
 	ParagraphMetrics const & pm = *pp.second;
 	pit_type const pit = pp.first;

status.22x

@@ -91,6 +91,8 @@ What's new
 
 - Fix random crash when dissolving inset (bug 10667).
 
+- Avoid crash when selecting long text (bug 10324).
+
 - Avoid a case of stuck cursor after entering an inset (bug 10630).