[1.9.x] Fixed #26018 -- Prevented unecessary get_form() call in FormMixin.get_context_data().

Changed "dict.setdefault" to "if x in dict" pattern so that get_form() would not
be called unnecessarily, specifically in the case where FormMixin.form_invalid()
calls get_context_data() with the current form.

Backport of e429c518 from master