mpmod aliasing

[Julius Schmidt <aiju@phicode.de>]

mpmod has a bug when n==r and x is negative.
my best attempt at a fix:

void
mpmod(mpint *x, mpint *n, mpint *r)
{
 	int sign;
 	mpint *ns;

 	sign = x->sign;
 	ns = sign < 0 && n == r ? mpcopy(n) : n;
 	if((n->flags & MPfield) == 0
 	|| ((Mfield*)n)->reduce((Mfield*)n, x, r) != 0)
 		mpdiv(x, n, nil, r);
 	if(sign < 0)
 		mpmagsub(ns, r, r);
 	if(ns != n) mpfree(ns);
}