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

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

diff --cc src/ecmult_const_impl.h
index b599316b86adb9fbdf7c25964ce0c6efa90e7ec8,61a47701d8d5ff78fe740848a4ec5054d7b11675..aaa576ada4cc22bb2bb3a784d0c2514e8aeccead
--- 1/src/ecmult_const_impl.h
--- 2/src/ecmult_const_impl.h
+++ b/src/ecmult_const_impl.h
@@@ -59,8 -59,12 +59,12 @@@ static int secp256k1_wnaf_const(int *wn
  
      int flip;
      int bit;
 -    secp256k1_scalar neg_s;
 +    secp256k1_scalar s;
      int not_neg_one;
+ 
+     VERIFY_CHECK(w > 0);
+     VERIFY_CHECK(size > 0);
+ 
      /* Note that we cannot handle even numbers by negating them to be odd, as is
       * done in other implementations, since if our scalars were specified to have
       * width < 256 for performance reasons, their negations would have width 256