MDEV-6083 - Assertion `! (&(&LOCK_open)->m_mutex)->count ||

            ! pthread_equal(pth read_self(), (&(&LOCK_open)->m_mutex)->thread)'
            fails in intern_sys_var_ptr on server shutdown after uninstalling
            TokuDB plugin at runtime

This assertion was introduced by patch for MDEV-5089 to ensure proper lock order
among LOCK_open and LOCK_global_system_variables: LOCK_open must not be held
while acquiring LOCK_global_system_variables.

intern_sys_var_ptr() may be called while freeing storage engine variables with
PLUGIN_VAR_MEMALLOC flag (when destroying table share after storage engine was
uninstalled). In this case LOCK_open is held, which is harmless because we need
global value pointer and thus won't acquire LOCK_global_system_variables.

Relaxed assertion so it is valid only for session variables.

sql/sql_plugin.cc

@@ -2900,11 +2900,12 @@ static uchar *intern_sys_var_ptr(THD* thd, int offset, bool global_lock)
   DBUG_ENTER("intern_sys_var_ptr");
   DBUG_ASSERT(offset >= 0);
   DBUG_ASSERT((uint)offset <= global_system_variables.dynamic_variables_head);
-  mysql_mutex_assert_not_owner(&LOCK_open);
 
   if (!thd)
     DBUG_RETURN((uchar*) global_system_variables.dynamic_variables_ptr + offset);
 
+  mysql_mutex_assert_not_owner(&LOCK_open);
+
   /*
     dynamic_variables_head points to the largest valid offset
   */