dup2 mingw failure

[Simon Josefsson]

I got this on MinGW:

test-dup2.c:140: assertion failed
FAIL: test-dup2.exe

The relevant code is:

  /* The destination must be valid.  */
  errno = 0;
  ASSERT (dup2 (fd, -2) == -1);

Debugging this, it seems MinGW dup2 returns -2 in this situation.  Any
ideas?  Is -2 a valid file descriptor on MinGW for some reason?

/Simon