Merge #841: Avoids a potentially shortening size_t to int cast in strauss_wnaf_

Merge #841: Avoids a potentially shortening size_t to int cast in strauss_wnaf_

8893f42438ac75838a9dc7df7e98b29e9a1a085f Avoids a potentially shortening size_t to int cast in strauss_wnaf_ (Tim Ruffing)

Pull request description:

ACKs for top commit:
  sipa:
    ACK 8893f42438ac75838a9dc7df7e98b29e9a1a085f. `np` and `no` shouldn't ever take on negative values.
  jonasnick:
    ACK 8893f42438ac75838a9dc7df7e98b29e9a1a085f
  elichai:
    ACK 8893f42438ac75838a9dc7df7e98b29e9a1a085f

Tree-SHA512: 431a6b88c8db8c8883b35c9bc03c90e37ecd0b06c7ee01c5d83cca4a7f6fc1f3cfbbaa871a4a23374ce4cc5bcfb9502c7f2e2540f9f9db9535e47e48827b6af6