create_sidecar_cb: fix/clarify error paths
create_sidecar_cb: fix/clarify error paths

• Coverity didn't realise that if
  gabble_plugin_loader_create_sidecar_finish() returned NULL, 'error'
  would be non-NULL. (Strictly speaking, it might not be, if the former
  is buggy or we passed it an invalid result.) So make this assumption
  explicit.
• If error == NULL, then calling g_clear_error (&error) will call
  g_error_free (NULL) which criticals.

Ziemin/telepathy-gabble,jku/telepathy-gabble,jku/telepathy-gabble,mlundblad/telepathy-gabble,mlundblad/telepathy-gabble,jku/telepathy-gabble,mlundblad/telepathy-gabble,Ziemin/telepathy-gabble,Ziemin/telepathy-gabble,Ziemin/telepathy-gabble