Fixed a bug that crasehed the server

when Stonegolems created a new building
and the whole unite was consumed during this process
(could not enter the new building as no one was left in the unit)

src/kernel/build.c

@@ -774,7 +774,8 @@ build_building(unit * u, const building_type * btype, int want, order * ord)
     fset(b, BLD_MAINTAINED);
 
     /* Die Einheit befindet sich automatisch im Inneren der neuen Burg. */
-    if (leave(u, false)) {
+    /* Check if unit still have member, as stonegolems are destroyed during construction*/
+    if (leave(u, false) && u->number >> 0) {     
       u_set_building(u, b);
       assert(building_owner(b)==u);
     }