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.

Approved by: re (cperciva)
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

(cherry picked from commit 4feeca3193c316c18472cd40f215446fda3bd739)
(cherry picked from commit 771a65210018c68198ea413df10225e87a0ea909)