pci-assign: Fix potential read beyond buffer on -EBUSY

readlink() doesn't write a terminating null byte.
assign_failed_examine() passes the unterminated string to strrchr().
Oops.  Terminate it.

Spotted by Coverity.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
