MDEV-8280: crash in 'show global status' with --skip-grant-tables

The crash was caused by attempting to use a hashtable without
it getting initialized, due to the --skip-grant-tables switch.

mysql-test/r/skip_grants.result

@@ -80,3 +80,14 @@ End of 5.0 tests
 select no_such_function(1);
 ERROR 42000: FUNCTION test.no_such_function does not exist
 End of 5.1 tests
+show global status like 'Acl%';
+Variable_name	Value
+Acl_column_grants	0
+Acl_database_grants	0
+Acl_function_grants	0
+Acl_procedure_grants	0
+Acl_proxy_users	0
+Acl_role_grants	0
+Acl_roles	0
+Acl_table_grants	0
+Acl_users	0

mysql-test/t/skip_grants.test

@@ -132,3 +132,8 @@ select count(*) from information_schema.USER_PRIVILEGES;
 select no_such_function(1);
 
 --echo End of 5.1 tests
+
+#
+# MDEV-8280 crash in 'show global status' with --skip-grant-tables
+#
+show global status like 'Acl%';

sql/sql_acl.cc

@@ -10383,11 +10383,14 @@ static int show_column_grants(THD *thd, SHOW_VAR *var, char *buff,
   var->type= SHOW_ULONG;
   var->value= buff;
   *(ulong *)buff= 0;
-  mysql_rwlock_rdlock(&LOCK_grant);
-  mysql_mutex_lock(&acl_cache->lock);
-  my_hash_iterate(&column_priv_hash, count_column_grants, buff);
-  mysql_mutex_unlock(&acl_cache->lock);
-  mysql_rwlock_unlock(&LOCK_grant);
+  if (initialized)
+  {
+    mysql_rwlock_rdlock(&LOCK_grant);
+    mysql_mutex_lock(&acl_cache->lock);
+    my_hash_iterate(&column_priv_hash, count_column_grants, buff);
+    mysql_mutex_unlock(&acl_cache->lock);
+    mysql_rwlock_unlock(&LOCK_grant);
+  }
   return 0;
 }