unsetenv test failure on openbsd 4.7

[Jim Meyering]

FYI,
No big deal, and no rush for such a corner case...
This certainly doesn't matter to grep.

While testing for grep, I hit this:

  $ ./test-unsetenv
  test-unsetenv.c:49: assertion failed
  Abort trap (core dumped)
  [Exit 134 (SIGABRT)]

It failed on this line:

  ASSERT (putenv ("") == -1);

The actual return value is 0.