Fix default minimum value for descending sequences

For some reason that is lost in history, a descending sequence would
default its minimum value to -2^63+1 (-PG_INT64_MAX) instead of
-2^63 (PG_INT64_MIN), even though explicitly specifying a minimum value
of -2^63 would work.  Fix this inconsistency by using the full range by
default.

Reported-by: Daniel Verite <daniel@manitou-mail.org>
Reviewed-by: Michael Paquier <michael.paquier@gmail.com>