tail(1): Fix -r (reverse) to work on pseudo filesystems

Pseudo filesystems (e.g., procfs) advertise a zero file size.
Fix reverse() to handle such a case similarly as forward() so
that '-r' works on pseudo filesystems.

Signed-off-by: Aaron LI <aly@aaronly.me>
Reviewed by: pouria, Ricardo Branco <rbranco@suse.de>, des
Fixes: 1fb3caee7 ("tail: Do not trust st_size if it equals zero.")
Pull-Request: https://github.com/freebsd/freebsd-src/pull/2080