libmount: initialize *root to NULL in mnt_table_get_root_fs

mnt_table_get_root_fs only works when *root is set to NULL. This
is not only undocumented, but also unintuitive. Fix it by initializing
*root inside mnt_table_get_root_fs.

Signed-off-by: Karel Zak <kzak@redhat.com>