wispr: Free wispr portal context in the relevant place

wispr: Free wispr portal context in the relevant place

This fixes 2 issues:
- calling __connman_service_ipconfig_indicate_state() might lead to
  restart wispr_portal check, so context should not be freed afterward
  but beforehand.
- freeing the context in wispr_manage_message() should not happen since
  wispr_manage_message() will return back to wispr_portal_web_result()
  where we can still use the context.

Thanks to Julien Massot <jmassot@aldebaran-robotics.com> who reported
the issue and provided logs.