Merge branch 'dd/mailinfo-with-nul'

Tighten "git mailinfo" to notice and error out when decoded result
contains NUL in it.

* dd/mailinfo-with-nul:
  mailinfo: disallow NUL character in mail's header
  mailinfo.c: avoid strlen on strings that can contains NUL
  t4254: merge 2 steps of a single test