mkswap: do not use uninitialized stack value

mkswap: do not use uninitialized stack value

If blkdev_get_size fails, then size is not set. Exit with an error code
and indicate what went wrong instead of continuing with random stack
content.

Proof of Concept:

```
$ mkswap /dev/null
mkswap: warning: truncating swap area to 17179869180 KiB
mkswap: /dev/null: insecure permissions 0666, fix with: chmod 0600 /dev/null
mkswap: unable to assign device to libblkid probe
```

The first output line depends on stack content and sometimes does not
show up at all. Abort operation if argument is neither regular file nor
block device.

Signed-off-by: Samanta Navarro <ferivoz@riseup.net>