Debugger: Correctly unset LocationMark when the editor is closed.

Change-Id: Ib178db5fd7d0e5674326287a471daf0e98a706d9
Reviewed-by: Niels Weber <niels.weber@theqtcompany.com>

src/plugins/debugger/debuggerengine.h

@@ -460,6 +460,7 @@ class LocationMark : public TextEditor::TextMark
 {
 public:
     LocationMark(DebuggerEngine *engine, const QString &file, int line);
+    void removedFromEditor() override { updateLineNumber(0); }
 
 private:
     bool isDraggable() const;