ccmake: Free long message form before its field

free_field() refuses to free a field that is still connected to a form
and returns E_CONNECTED.  ~cmCursesLongMessageForm freed its field
before the base class destructor freed the form, so the field and its
buffer leaked every time a long message form was destroyed.

Source/CursesDialog/cmCursesLongMessageForm.cxx

@@ -30,6 +30,12 @@ cmCursesLongMessageForm::cmCursesLongMessageForm(
 
 cmCursesLongMessageForm::~cmCursesLongMessageForm()
 {
+  // The field cannot be freed while it is connected to the form
+  if (this->Form) {
+    unpost_form(this->Form);
+    free_form(this->Form);
+    this->Form = nullptr;
+  }
   if (this->Fields[0]) {
     free_field(this->Fields[0]);
   }