[freeciv-ai] Bug: Assert in aiferry.c

[Jordi Negrevernis i Font <jorneg@xxxxxxxxxxx>]

   The line:

   handle_unit_load(pplayer, punit->id, ferryboat->id);
   assert(punit->transported_by > 0);
 }

   Asserts a lot in current cvs!

   It can happen that the move fails. So, it will great if there would 
be only a message and a return like in the paragraph above.