set queue to NULL

the kernel driver has the following check in gckCOMMAND_Commit(..)

gceSTATUS
gckCOMMAND_Commit(
    IN gckCOMMAND Command,
    IN gckCONTEXT Context,
    IN gcoCMDBUF CommandBuffer,
    IN gcsSTATE_DELTA_PTR StateDelta,
    IN gcsQUEUE_PTR EventQueue,
    IN gctUINT32 ProcessID
    )
{

..

    /* Loop while there are records in the queue. */
    while (EventQueue != gcvNULL)
..
}

so for the moment it is safe to do it this way.