TW-1678: segfault in ~ViewTask()

- Column::setStyle now validates column styles.
- Thanks to Daniel Shahaf.

src/columns/ColDue.cpp

@@ -52,7 +52,7 @@ ColumnDue::~ColumnDue ()
 // Note that you can not determine which gets called first.
 void ColumnDue::setStyle (const std::string& value)
 {
-  _style = value;
+  Column::setStyle (value);
 
   if (_style == "countdown" && _label == STRING_COLUMN_LABEL_DUE)
     _label = STRING_COLUMN_LABEL_COUNT;