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 pcmCommand in GetCommand
Merge pull request #4940

62e5f8f CMessageHeader sanity changes (Wladimir J. van der Laan)