bearer-mbim: fix disconnect error handling case split

The first case, with no error, needs to check that there was no original error.
Currently, if an error occurs and the resulting NwError is parsed correctly, the
case split treats it as a no-error case.

src/mm-bearer-mbim.c

@@ -980,7 +980,7 @@ disconnect_set_ready (MbimDevice *device,
         }
 
         /* Now handle different response / error cases */
-        if (parsed_result) {
+        if (result && parsed_result) {
             mm_dbg ("Session ID '%u': %s",
                     session_id,
                     mbim_activation_state_get_string (activation_state));