Fix crash when sorting one table in the Browse Tab and changing to another

Fix a crash when sorting the n-th column of a table in the Browse Tab
and then changing to a table with less than n columns.

src/sqlitetablemodel.h

@@ -11,6 +11,7 @@ class SqliteTableModel : public QAbstractTableModel
     Q_OBJECT
 public:
     explicit SqliteTableModel(QObject *parent = 0, DBBrowserDB* db = 0, size_t chunkSize = 50000);
+    void reset();
 
     int rowCount(const QModelIndex &parent = QModelIndex()) const;
     int totalRowCount() const;