interface: report correct interface count when not returning list

The spec for virConnectListAllInterfaces says that if the pointer that
is supposed to hold the list of interfaces is NULL, the function
should just return the count of interfaces that matched the filter,
but the code never increments the count if the list pointer is NULL.

src/interface/interface_backend_netcf.c

@@ -632,8 +632,9 @@ netcfConnectListAllInterfaces(virConnectPtr conn,
             if (!(iface_obj = virGetInterface(conn, ncf_if_name(iface),
                                               ncf_if_mac_string(iface))))
                 goto cleanup;
-            tmp_iface_objs[niface_objs++] = iface_obj;
+            tmp_iface_objs[niface_objs] = iface_obj;
         }
+        niface_objs++;
 
         ncf_if_free(iface);
         iface = NULL;