MDEV-28773 SIGSEGV in TABLE::use_all_columns, replace_roles_mapping_table

as elsewhere, it a priv table doesn't exist - skip it
only update in-memory structures

Author: vuvova

mysql-test/main/grant_repair.result

@@ -80,4 +80,11 @@ grant proxy on grant_plug to grant_plug_dest;
 ERROR HY000: Cannot load from mysql.proxies_priv. The table is probably corrupted
 drop table mysql.proxies_priv;
 rename table mysql.proxies_priv_bak to mysql.proxies_priv;
+#
+# MDEV-28773 SIGSEGV in TABLE::use_all_columns, replace_roles_mapping_table
+#
+rename table mysql.roles_mapping to mysql.roles_mapping_bak;
+create role r1;
+drop role r1;
+rename table mysql.roles_mapping_bak to mysql.roles_mapping;
 # End of 10.11 tests

mysql-test/main/grant_repair.test

@@ -96,4 +96,12 @@ grant proxy on grant_plug to grant_plug_dest;
 drop table mysql.proxies_priv;
 rename table mysql.proxies_priv_bak to mysql.proxies_priv;
 
+--echo #
+--echo # MDEV-28773 SIGSEGV in TABLE::use_all_columns, replace_roles_mapping_table
+--echo #
+rename table mysql.roles_mapping to mysql.roles_mapping_bak;
+create role r1;
+drop role r1;
+rename table mysql.roles_mapping_bak to mysql.roles_mapping;
+
 --echo # End of 10.11 tests

sql/sql_acl.cc

@@ -11261,11 +11261,11 @@ bool mysql_create_user(THD *thd, List <LEX_USER> &list, bool handle_as_role)
 
       /* TODO(cvicentiu) refactor replace_roles_mapping_table to use
          Roles_mapping_table instead of TABLE directly. */
-      if (replace_roles_mapping_table(tables.roles_mapping_table().table(),
+      if (tables.roles_mapping_table().table_exists() &&
+          replace_roles_mapping_table(tables.roles_mapping_table().table(),
                                       &thd->lex->definer->user,
                                       &thd->lex->definer->host,
-                                      &user_name->user, true,
-                                      NULL, false))
+                                      &user_name->user, true, NULL, false))
       {
         append_user(thd, &wrong_users, user_name);
         if (grantee)