pgsql: 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: https://postgr.es/m/1618848d-cdc7-414b-9c03-08cf4bef4408@postgrespro.ru
Backpatch-through: 13

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/67a02341572e0dcf27a290953317a39c7f15cc82

Modified Files
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src/backend/executor/execMain.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)