get_termkey can segfault from accessing NULL

[Anthony DeRobertis]

I've stumbled across this while running the POEx::Tickit test cases under Debhelper, which appears to have done something unexpected — redirected stdin from /dev/null.

static TermKey *get_termkey(TickitTerm *tt)
{
  if(!tt->termkey) {
    int flags = 0;
    if(tt->is_utf8 == TICKIT_YES)
      flags |= TERMKEY_FLAG_UTF8;
    else if(tt->is_utf8 == TICKIT_NO)
      flags |= TERMKEY_FLAG_RAW;

    tt->termkey = termkey_new(tt->infd, TERMKEY_FLAG_EINTR | flags);

    tt->is_utf8 = !!(termkey_get_flags(tt->termkey) & TERMKEY_FLAG_UTF8);
  }
  ⋮

The problem is that termkey_new can fail (in this case because termkey_start failed), and thus return NULL. Then the next line tries to access tt->termkey, i.e., NULL->termkey, which of course segfaults.

Unfortunately, the fix appears non-trivial as other places seem to presume get_termkey never fails.