libmount: fix undefined sources

libmount: fix undefined sources

The path resolving function breaks when handling undefined sources
("none") used by pseudo-filesystems. The assertion is causing the
samples/mount program to abort:

dave@offbook:~/projects/util-linux-good/libmount/samples$ ./mount
/dev/sda3 on / type ext4 (rw,errors=remount-ro,commit=0)
proc on /proc type proc (rw,noexec,nosuid,nodev)
lt-mount: cache.c:464: mnt_resolve_path: Assertion `path' failed.
Aborted

/etc/mtab:

/dev/sda3 / ext4 rw,errors=remount-ro,commit=0 0 0
proc /proc proc rw,noexec,nosuid,nodev 0 0
none /sys sysfs rw,noexec,nosuid,nodev 0 0
fusectl /sys/fs/fuse/connections fusectl rw 0 0
none /sys/kernel/debug debugfs rw 0 0
...

Instead of this assert on NULL we can simply return NULL and let mnt_pretty_path do the work.

Signed-off-by: Davidlohr Bueso <dave@gnu.org>