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libmp: fix mpmod() aliasing bug when n == r and x < 0 (thanks aiju, mischief)

mischief found this in rsafill()'s call mpmod(c2, x, x), where
d parameter is negative (rsagen created a rsa key with negative dk).

--- a/sys/src/libmp/port/mpmod.c

+++ b/sys/src/libmp/port/mpmod.c

@@ -6,11 +6,15 @@

 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(n, r, r);

+	if(sign < 0){

+		mpmagsub(ns, r, r);

+		if(ns != n) mpfree(ns);

+	}

 }