Merge #629: Avoid calling _is_zero when _set_b32 fails.

cd473e0 Avoid calling secp256k1_*_is_zero when secp256k1_*_set_b32 fails. (Gregory Maxwell)

Pull request description:

  Most of the codebase correctly used short-cutting to avoid calling
   _is_zero on possibly incompletely initialized elements, but a few
   places were missed.

ACKs for commit cd473e:
  sipa:
    utACK cd473e02c372217c3a6608ce5afaa543ed78f891
  jonasnick:
    utACK cd473e02c372217c3a6608ce5afaa543ed78f891

Tree-SHA512: d6af2863f6795d2df26f2bd05a4e33085e88c45f7794601ea57e67238a2073ef1ee3ba0feab62a7fcbc0636c48dfd80eea07d0ca4f194414127f914b0478c732

diff --cc src/secp256k1.c
Simple merge