MDEV-20257 Server crashes in Grant_table_base::init_read_record upon crash-upgrade

when opening the `user` table separately, reset `thd->open_tables`
for the duration of open, otherwise auto-repair fallback-and-retry
will close *all* tables (but reopen only `user`)

mysql-test/main/grant_repair.result

+call mtr.add_suppression("/mysql/user");
+# switching from mysql.global_priv to mysql.user
+flush tables;
+flush privileges;
+Warnings:
+Error	145	Table './mysql/user' is marked as crashed and should be repaired
+Warning	1034	12544 clients are using or haven't closed the table properly
+Note	1034	Table is fixed
+# switching back from mysql.user to mysql.global_priv

mysql-test/main/grant_repair.test

+#
+# MDEV-20257 Server crashes in Grant_table_base::init_read_record upon crash-upgrade
+#
+
+call mtr.add_suppression("/mysql/user");
+source include/switch_to_mysql_user.inc;
+
+let mysql_datadir=`select @@datadir`;
+flush tables;
+perl;
+  $f=$ENV{mysql_datadir} . '/mysql/user.MAI';
+  sysopen F, $f, 2  or die "sysopen $f: $!";
+  # set the open_count to 1, "table was not closed properly"
+  sysseek F, 24, O_RDWR or die "sysseek: $!";
+  syswrite F, 1, 1 or die "syswrite: $!";
+EOF
+flush privileges;
+
+source include/switch_to_mysql_global_priv.inc;

sql/sql_acl.cc

@@ -1847,10 +1847,13 @@ class Grant_tables
     int res= really_open(thd, first, &counter);
 
     /* if User_table_json wasn't found, let's try User_table_tabular */
-    if (!res && (which_tables & Table_user) && !(tables[USER_TABLE].table))
+    if (!res && (which_tables & Table_user) && !tables[USER_TABLE].table)
     {
       uint unused;
       TABLE_LIST *tl= tables + USER_TABLE;
+      TABLE *backup_open_tables= thd->open_tables;
+      thd->set_open_tables(NULL);
+
       tl->init_one_table(&MYSQL_SCHEMA_NAME, &MYSQL_TABLE_NAME_USER,
                          NULL, lock_type);
       tl->open_type= OT_BASE_ONLY;
@@ -1859,6 +1862,12 @@ class Grant_tables
       p_user_table= &m_user_table_tabular;
       counter++;
       res= really_open(thd, tl, &unused);
+      thd->set_open_tables(backup_open_tables);
+      if (tables[USER_TABLE].table)
+      {
+        tables[USER_TABLE].table->next= backup_open_tables;
+        thd->set_open_tables(tables[USER_TABLE].table);
+      }
     }
     if (res)
       DBUG_RETURN(res);