Any person who threatens to publish a libel concerning any other
person, or concerning any relative, wife or child or dead relative of
such person, or member of his family, shall be liable civilly and
criminally to have the same intent as though the publication had been
made. But if the threat be not in writing, the threat and character of
the libelous matter must be proven by at least two witnesses, or by
one witness and corroborating circumstances.