Set output coordinates' |neg| field in |ecp_nistz256_get_affine|.

The result would not be correct if, on input, |x->neg != 0| or
|y->neg != 0|.

Change-Id: I645566a78c2e18e42492fbfca1df17baa05240f7
Reviewed-on: https://boringssl-review.googlesource.com/7587
Reviewed-by: David Benjamin <davidben@google.com>

crypto/ec/p256-x86_64.c

@@ -528,6 +528,7 @@ static int ecp_nistz256_get_affine(const EC_GROUP *group, const EC_POINT *point,
       return 0;
     }
     x->top = P256_LIMBS;
+    x->neg = 0;
     ecp_nistz256_from_mont(x->d, x_aff);
     bn_correct_top(x);
   }
@@ -540,6 +541,7 @@ static int ecp_nistz256_get_affine(const EC_GROUP *group, const EC_POINT *point,
       return 0;
     }
     y->top = P256_LIMBS;
+    y->neg = 0;
     ecp_nistz256_from_mont(y->d, y_aff);
     bn_correct_top(y);
   }