MDEV-26796 Natural sort does not work for utf32/utf16/ucs2

Fixed typo, added test.

mysql-test/main/natural_sort_key.result

@@ -203,3 +203,6 @@ NULL	NULL
 a2	a02
 a11	a111
 drop table t;
+select natural_sort_key(_utf16 0x0031),natural_sort_key(_ucs2 0x0031), natural_sort_key(_utf32 0x00000031);
+natural_sort_key(_utf16 0x0031)	natural_sort_key(_ucs2 0x0031)	natural_sort_key(_utf32 0x00000031)
+01	01	01

mysql-test/main/natural_sort_key.test

@@ -93,3 +93,5 @@ select * from t order by b;
 select a, b from t order by b;
 drop table t;
 
+# MDEV-26796 Natural sort does not work for utf32/utf16/ucs2
+select natural_sort_key(_utf16 0x0031),natural_sort_key(_ucs2 0x0031), natural_sort_key(_utf32 0x00000031);

sql/item_strfunc.cc

@@ -5657,7 +5657,7 @@ String *Item_func_natural_sort_key::val_str(String *out)
   */
   if (cs->mbminlen != 1)
   {
-    if (!tmp.copy(in, &my_charset_utf8mb4_bin, &errs))
+    if (tmp.copy(in, &my_charset_utf8mb4_bin, &errs))
       goto error_exit;
     in= &tmp;
   }