Load the correct ATS plugin when MODE is unconfigured.

In this case ah->ats_mode was set to MODE_PROPORTIONAL
but plugin_short was not touched. Instead initialize
plugin_short after we are done selecting ah->ats_mode.
This will be less prone to regression when a change
is made to the ats_mode selection code.


git-svn-id: https://gnunet.org/svn/gnunet@31765 140774ce-b5e7-0310-ab8b-a85725594a96