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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REL_14_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/971566e9650be7defdafa19b6112408233ca3c6b

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