BUG #6097: Server crash when enabling custom_variable_classes

The following bug has been logged online:

Bug reference: 6097
Logged by: Maxim Boguk
Email address: Maxim(dot)Boguk(at)gmail(dot)com
PostgreSQL version: 9.0 9.1beta
Operating system: Linux/Freebsd
Description: Server crash when enabling custom_variable_classes
Details:

I found crash bug in custom_variable_classes work.

Test case simple:
start server with empty custom_variable_classes

Then add to config file:

custom_variable_classes = 'something' # list of custom
variable class names
something.location = 'ru'

And then perform
select pg_reload_conf();

Ooops... you have no more working server (but client connections staying
alive).

Same crash with 9.0 and 9.1beta.

Adding just
custom_variable_classes = 'qqqq'
and pg_reload_conf() do not crash server.
But after I added qqqq.location = 'ru' to config and perform second
pg_reload_conf() server crashed again.

The same happens if server started with just custom_variable_classes =
'qqqq',
after I added qqqq.location = 'ru' and performend pg_reload_conf() server
crashed.

However with both option enabled server start and work without any issues.

PS: it is exeptionally strange no one happen to be hit by that problem
before.