Fix formatting of Interval output

When formatting the output of an Interval, we call abs() on the hours
field of the Interval. Calling abs(INT_MIN) returns back INT_MIN
causing the output to contain two '-' characters. The attached patch
fixes that issue by special casing INT_MIN hours.

Here is an example of the issue:
postgres=# SELECT INTERVAL '-2147483648 hrs';
interval
--------------------
--2147483648:00:00
(1 row)

Cheers,
Joe Koshakow