storage: rbd: do not attempt to dereference a non-pointer

My commit 9b7c4048 was too blind
and my librbd was not new enough to actually compile this part.
Signed-off-by: NJán Tomko <jtomko@redhat.com>

src/storage/storage_backend_rbd.c

@@ -637,7 +637,7 @@ virStorageBackendRBDGetVolNames(virStorageBackendRBDStatePtr ptr)
     nnames = nimages;
 
     for (i = 0; i < nimages; i++)
-        VIR_STEAL_PTR(names[i], images[i]->name);
+        VIR_STEAL_PTR(names[i], images[i].name);
 
     return names;