! 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.