Make sure to
- append a trailing \0 as with Windows multistrings, so the end of
the string can be recognized by the caller, and
- allocate cmdline on the user heap so the caller can free the
multistring after usage.
Fixes: 831d6fa520 ("* external.cc (cygwin_internal): Implement CW_CMDLINE.")
Signed-off-by: Corinna Vinschen <corinna@vinschen.de>