fixes issue #6119

[joe-hauns]

Makes sure that two equal integers always have the same hash value, even in corner cases where they are represented differently.
The issue observed was that the integer -2147483647 was hashed to different values depending on being represented as small or large integer.
In the small case it used to be hashed to unsigned(-2147483647) which is 2147483649, and in the large case it was hashed to 2147483647, as the digits are all represented as unsigneds anyways.