Fixup for MDEV-18968

Item_cond::eval_not_null_tables(): Use Item::eval_const_cond(),
just like Item_cond::fix_fields().

This inconsistency was found while merging to 10.3, where the
Microsoft compiler is configured to report an error for comparing
longlong to bool.

sql/item_cmpfunc.cc

 /* Copyright (c) 2000, 2013, Oracle and/or its affiliates.
-   Copyright (c) 2009, 2016, MariaDB
+   Copyright (c) 2009, 2019, MariaDB
 
    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
@@ -4728,7 +4728,7 @@ Item_cond::eval_not_null_tables(uchar *opt_arg)
     if (item->const_item() && !item->with_param &&
         !item->is_expensive() && !cond_has_datetime_is_null(item))
     {
-      if (item->val_int() == is_and_cond && top_level())
+      if (item->eval_const_cond() == is_and_cond && top_level())
       {
         /* 
           a. This is "... AND true_cond AND ..."