iommu/amd: Check if domain is NULL in get_domain() and return -EBUSY

In get_domain(), 'domain' could be NULL before it's passed to dma_ops_domain() to dereference. And the current code calling get_domain() can't deal with the returned 'domain' well if its value is NULL.

So before dma_ops_domain() calling, check if 'domain' is NULL, If yes just return ERR_PTR(-EBUSY) directly.

---
 drivers/iommu/amd_iommu.c | 3 +++
 1 file changed, 3 insertions(+)

index 16f1e6af00b0..2d7d04472555 100644
@@ -2262,6 +2262,9 @@ static struct protection_domain *get_domain(struct device *dev)
 		domain = to_pdomain(io_domain);
 		attach_device(dev, domain);
 	}
+	if (domain == NULL)
+		return ERR_PTR(-EBUSY);
+
 	if (!dma_ops_domain(domain))
 		return ERR_PTR(-EBUSY);
 

2.5.5