(PR#17884) Assert on SVN HEAD on advent of Future Technologies

<URL: 
http://bugs.freeciv.org/Ticket/Display.html?id=17884
>


    I did modify choose_tech() to skip A_UNSET, because 
ai_manage_techs() sends him a A_UNSET value returned by
ai_select_tech() 
when the ai has finished the tech tree... so it asserts on
get_invention!!

    Thanks


    PS: I don't know if this happens in S2_1.


--- techtools.c.orig	2006-06-09 13:22:36.000000000 +0200
+++ techtools.c	2006-06-09 15:00:36.000000000 +0200
 -526,6
+526,10 
   if (research->researching == tech) {
     return;
   }
+  if (tech == A_UNSET) {
+    /* we are beginning to research new technologies */
+    return;
+  }
   if (get_invention(plr, tech) != TECH_REACHABLE) {
     /* can't research this */
     return;