CMessageHeader sanity changes

- Remove spurious `pchCommand[1] = 1` in CMessageHeader()
- Make sure that pchCommand is zero-padded if length is shorter than COMMAND_SIZE
- Use strnlen to determine length of pchCommand in GetCommand

I'm afraid that pulltester's old mingw will complain about lack of `strnlen`. This is not actually a problem with more recent compilers on windows: I verified that it works fine with gitian's version, and it should work with Travis.