Trac 16807: fix overflow in conversion Integer -> FiniteFieldElement_pari_ffelt
Trac #16807: Overflow error in conversion Integer -> FiniteFieldElement_pari_ffelt

As reported by Samuel Neves on [https://groups.google.com/forum/#!topic
/sage-support/ARJ3-5EFhrs sage-support], the following raises a PARI
error due to an overflow in converting from `t_INT` to long:
{{{
p = previous_prime(2^64)
F.<x> = GF(p^2)
x * 2**63
}}}
The problem is that the conversion is done using PARI's `itos` (`t_INT`
to signed long) instead of `itou` (`t_INT` to unsigned long).

URL: http://trac.sagemath.org/16807
Reported by: pbruin
Ticket author(s): Peter Bruin
Reviewer(s): Jeroen Demeyer