Fix assertion on dereferenced object

Fix assertion on dereferenced object

Commit 27cc7cd2bc8a accidentally placed the assertion ensuring
that the pointer isn't NULL after it had already been accessed.
Fix by moving the pointer dereferencing to after the assertion.
Backpatch to all supported branches.

Author: Dmitry Koval <d.koval@postgrespro.ru>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Discussion: http://postgr.es/m/1618848d-cdc7-414b-9c03-08cf4bef4408@postgrespro.ru
Backpatch-through: 13