utils.walk_together() is incorrect

[cariaso]

Although I actually found the issue in the https://github.com/sein-tao/cyvcf fork, that one doesn't allow issues, while this one seems no longer active.

issue occurs when 2 files are being walked, and both have values at the same position. They are returned in 2 different yields ie yield (rec1, None) yield (None,rec2)

instead of being a single yield of yield (rec1, rec2)