Fix misbehavior in contrib/pg_trgm with an unsatisfiable regex.

If the regex compiler can see that a regex is unsatisfiable
(for example, '$foo') then it may emit an NFA having no arcs.
pg_trgm's packGraph function did the wrong thing in this case;
it would access off the end of a work array, and with bad luck
could produce a corrupted output data structure causing more
problems later.  This could end with wrong answers or crashes
in queries using a pg_trgm GIN or GiST index with such a regex.

Fix by not trying to de-duplicate if there aren't at least 2 arcs.

Per bug #17830 from Alexander Lakhin.  Back-patch to all supported
branches.

Discussion: https://postgr.es/m/17830-57ff5f89bdb02b09@postgresql.org

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