Rejection of the smallest int8

Attached is a patch to accept the smallest value of int8.

The smallest value -9223372036854775808 is rejected as follows:

test=# create table test_int8 (val int8);
CREATE
test=# insert into test_int8 values (-9223372036854775807);
INSERT 4026531936 1
test=# insert into test_int8 values (-9223372036854775808);
ERROR: int8 value out of range: "-9223372036854775808"
test=#