tests/disks: Don't segfault if guestfs_list_devices returns an error.

Although it's highly unlikely in normal use, while testing device name
translation patches it did happen and caused the test to segfault
instead of exiting with an error.

tests/disks/test-add-disks.c

@@ -261,6 +261,8 @@ do_test (guestfs_h *g, size_t ndisks, bool just_add)
 
   /* Check the disks were added. */
   devices = guestfs_list_devices (g);
+  if (devices == NULL)
+    exit (EXIT_FAILURE);
   n = guestfs_int_count_strings (devices);
   if (n != ndisks) {
     fprintf (stderr, "%s: incorrect number of devices returned by guestfs_list_devices:\n",