Merge #568: Fix integer overflow in ecmult_multi_var when n is large

2277af5 Fix integer overflow in ecmult_multi_var when n is large (Jonas Nick)

Pull request description:

  Without this PR ecmult_multi could return wrong results. If the number of points `n` is large enough then some or all multiplications could be skipped or the function could end up in an infinite loop. This PR adds two checks to prevent `n` from wrapping around.

Tree-SHA512: 342944369b24776fa3ec0694eee159259ff67e94d2d8176c1d3159875f387d943d5bfdff7cde59f058e13f07fd09bde1cbc609426e63c8a5b8040e382dd865d8