e2fsck: handle null fs in print_pathname()

e2fsck: handle null fs in print_pathname()

testing fs for NULL in expand_percent_expression():

	e2fsck_ctx = fs ? (e2fsck_t) fs->priv_data : NULL;

implies that fs could be NULL, but it's passed to print_pathname()
which defererences it without further testing.

So make this safe by returning "???" for a nul fs.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>