iommufd: iommufd_hw_pagetable_dma_fault_destroy check header

If guest do not have device buildin,
iommufd_hwpt_alloc cmd->eventfd=-1
iommufd_hw_pagetable_dma_fault_init will not be called
s1->fault_pages is NULL
iommufd_hw_pagetable_dma_fault_destroy fail with NULL pointer

Signed-off-by: Zhangfei Gao <zhangfei.gao@linaro.org>

drivers/iommu/iommufd/hw_pagetable.c

@@ -25,6 +25,9 @@ iommufd_hw_pagetable_dma_fault_destroy(struct iommufd_hw_pagetable *hwpt)
 	struct iommufd_stage1_dma_fault *header =
 		(struct iommufd_stage1_dma_fault *)s1->fault_pages;
 
+	if (!header)
+		return;
+
 	WARN_ON(header->tail != header->head);
 	iommufd_hw_pagetable_eventfd_destroy(&s1->trigger);
 	kfree(s1->fault_pages);