mingw: test-ftell and test-ftello assertion failures

[Matthias Bolte]

In both tests ASSERT (ftell (stdin) == -1); in line 45 fails. ftell
(stdin) returns 0 in this case.

Found via libvirt on mingw with recent gnulib version:
f4c4af09bcf3f0497dc3347ecc0a0b3c9ee1ff63

Matthias