Use correct symbol for minimum int64 value

Use correct symbol for minimum int64 value

The old code used SEQ_MINVALUE to get the smallest int64 value.  This
was done as a convenience to avoid having to deal with INT64_IS_BUSTED,
but that is obsolete now.  Also, it is incorrect because the smallest
int64 value is actually SEQ_MINVALUE-1.  Fix by using PG_INT64_MIN.