Merge #533: Make sure we're not using an uninitialized variable in secp256k1_wnaf_con...

Merge #533: Make sure we're not using an uninitialized variable in secp256k1_wnaf_const(...)

248f046 Make sure we're not using an uninitialized variable in secp256k1_wnaf_const(...) (practicalswift)

Pull request description:

  Make sure we're not using an uninitialized variable in `secp256k1_wnaf_const(...)`:

  ```
  In file included from src/secp256k1.c:15:0,
                   from src/tests.c:17:
  src/ecmult_const_impl.h: In function ‘secp256k1_wnaf_const’:
  src/ecmult_const_impl.h:117:20: warning: ‘u’ may be used uninitialized in this function [-Wmaybe-uninitialized]
       wnaf[word] = u * global_sign;
                      ^
  ```

  **Note to reviewers:** Perhaps an `assert(…);` is a bit drastic. What would be a more graceful way to handle this? :-)

Tree-SHA512: 536cd7cc5b87a84fbaac578cecbba81b8d82e4672a30a2db9a674b82856132e79b0158a6a88609bc24942ebdbf1fcd2c4399a4c31ab0654b88ace9c0e6f1eaf3