Fix a segfault when calling api.dynobj_putvalues()

An undocumented change from the previous commit:
- When the Lua script is not able to find an object, the message is now printed to `errors.txt`, not just to `log.txt`.

scripting/lua.h

@@ -5,6 +5,7 @@
 class basic_event;
 class TMemCell;
 class TDynamicObject;
+struct memcell_values { const char *str; double num1; double num2; };
 
 class lua
 {
@@ -44,6 +45,5 @@ public:
 	static void unref(lua_State *L, int ref);
 	static void dispatch_event(lua_State *L, int handler, basic_event *event, const TDynamicObject *activator);
 	static void push_memcell_values(lua_State *L, const TMemCell *mc);
-
-	struct memcell_values { const char *str; double num1; double num2; };
+	static memcell_values get_memcell_values(lua_State *L);
 };