tests/hd-geo-test: Don't pass NULL to unlink()

The unlink() function doesn't accept a NULL pointer, so
don't pass it one. Spotted by the clang sanitizer.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: John Snow <jsnow@redhat.com>
Message-id: 1470391392-28274-1-git-send-email-peter.maydell@linaro.org