IPFlowID hashcode: Avoid 32-bit shift.

The shift isn't that dangerous (as long as the architecture always
treats 32-bit shifts the same way and doesn't throw), but fixing it is
better than not.

John Regehr report.
If x == INT_MIN, then prefer -(x + 1) (which is defined)

to -x - 1 (which isn't).
Avoid another signed integer overflow.

Reported by John Regehr.