Strange timezone 'math' in 9.6.8

Seems that PostgreSQL does do math inside invalid time zone names,
without reporting any errors to client if time zone name has sign and
number inside range.

staging=# show server_version;
server_version
----------------
9.6.8
(1 row)

staging=# select now(), now() at time zone 'fubar+167';
now | timezone
-------------------------------+----------------------------
2018-03-28 09:16:25.897444+00 | 2018-03-21 10:16:25.897444
(1 row)

staging=# select now(), now() at time zone 'fubar+168';
ERROR: time zone "fubar+168" not recognized

According to http://sqlfiddle.postgrespro.ru can be reproduced in
11devel and 10.3 (Ubuntu 10.3-1.pgdg16.04+1)