vhost-net: fix vq_memory_access_ok error checking

vq_memory_access_ok needs to check whether mem == NULL
Signed-off-by: NJeff Dike <jdike@linux.intel.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

drivers/vhost/vhost.c

@@ -235,6 +235,10 @@ static int vq_memory_access_ok(void __user *log_base, struct vhost_memory *mem,
 			       int log_all)
 {
 	int i;
+
+        if (!mem)
+                return 0;
+
 	for (i = 0; i < mem->nregions; ++i) {
 		struct vhost_memory_region *m = mem->regions + i;
 		unsigned long a = m->userspace_addr;