Add DataFilter destructor calling clearFilter to release memory

The crash documented in #4249 was related to the conversion of
Leaf of type Symbol to Function when () are found afterwards.
This is fixed in this commit, but more testing is necessary to
see if there are other cases.

src/Core/DataFilter.h

@@ -219,6 +219,7 @@ class DataFilter : public QObject
     public:
         DataFilter(QObject *parent, Context *context);
         DataFilter(QObject *parent, Context *context, QString formula);
+        ~DataFilter() { clearFilter(); }
 
         // runtime passed by datafilter
         DataFilterRuntime rt;

src/Core/DataFilter.y

@@ -458,6 +458,8 @@ expr:
                                                   $1->type = Leaf::Function;
                                                   $1->series = NULL; // not tiz/best
                                                   $1->function = *($1->lvalue.n);
+                                                  delete $1->lvalue.n; // not used anymore
+                                                  $1->lvalue.l = NULL; // avoid double deletion
                                                   $1->fparms.clear(); // no parameters!
                                                 }
         | '(' expr ')'                          { $$ = new Leaf(@2.first_column, @2.last_column);