Issue #317: check if spell actually exist in AddSpell

apps/openmw/mwscript/statsextensions.cpp

@@ -299,6 +299,9 @@ namespace MWScript
                     std::string id = runtime.getStringLiteral (runtime[0].mInteger);
                     runtime.pop();
 
+                    // make sure a spell with this ID actually exists.
+                    MWBase::Environment::get().getWorld()->getStore().spells.find (id);
+
                     MWWorld::Class::get (ptr).getCreatureStats (ptr).mSpells.add (id);
                 }
         };