json: do not call json_tokener_free with NULL

json: do not call json_tokener_free with NULL

Add an error message for the rare case if json_tokener_new
fails (allocation failure) and guard any use of json_tokener_free
where tok might be NULL (this was possible in libvirt-nss
when the json file could not be opened).

https://gitlab.com/libvirt/libvirt/-/issues/581

Signed-off-by: Ján Tomko <jtomko@redhat.com>
Reported-by: Simon Pilkington
Reviewed-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Richard W.M. Jones <rjones@redhat.com>