Explicitly return when a diplomat action query won't get the wanted information.

At the moment nothing is done after unit_query_impossible() is called. Add an
explicit return statement after all calls to it to make sure this won't change
by accident.

See patch #4653

git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@24770 a0f10bec-cc02-0410-94fc-a9cfff90b4cd