Fix core dump in udiskslinuxdriveobject.c

[cenhuil]

Ejecting and then plugging back the mobile hard disk causes udisks core dump

[tbzatek]

Thank you for the patch, however you're fixing a consequence, not a cause. Fixing it here will cover the root cause and will likely cause crash somewhere else. We need to find out why device == NULL for a given UDisksLinuxBlockObject. Is the problem reproducible? Can you try grabbing a full gdb stacktrace?