crash casued by spell conversion.

remove unit from faction, not only region!
also remove spammy warning that has no value.

src/kernel/save.c

@@ -1707,6 +1707,7 @@ int read_game(gamedata *data) {
 
             if (data->version < NORCSPELL_VERSION && u_race(u) == rc_spell) {
                 set_observer(r, u->faction, get_level(u, SK_PERCEPTION), u->age);
+                u_setfaction(u, NULL);
                 free_unit(u);
             }
             else {

src/kernel/unit.c

@@ -1400,10 +1400,12 @@ int invisible(const unit * target, const unit * viewer)
 /** remove the unit from memory.
  * this frees all memory that's only accessible through the unit,
  * and you should already have called uunhash and removed the unit from the
- * region.
+ * region and faction.
  */
 void free_unit(unit * u)
 {
+    assert(!u->region);
+    assert(!u->faction);
     free(u->_name);
     free(u->display);
     free_order(u->thisorder);

src/magic.c

@@ -3010,7 +3010,6 @@ int cast_spell(struct castorder *co)
 
     fun = get_spellcast(sp->sname);
     if (!fun) {
-        log_warning("no spell function for %s, try callback", sp->sname);
         return callbacks.cast_spell(co, fname);
     }
     return fun(co);