pgsql: Fix handling of previous password hooks in passwordcheck

Fix handling of previous password hooks in passwordcheck

When piling up loading of modules using check_password_hook_type,
loading passwordcheck would remove any trace of a previously-loaded
hook. Unloading the module would also cause previous hooks to be
entirely gone.

Reported-by: Rafael Castro
Author: Michael Paquier
Reviewed-by: Daniel Gustafsson
Discussion: /messages/by-id/15932-78f48f9ef166778c@postgresql.org
Backpatch-through: 9.4

Branch
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REL9_4_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/eea28a3cb36951ee439288d8e400621ec301484f

Modified Files
--------------
contrib/passwordcheck/passwordcheck.c | 20 ++++++++++++++++++++
1 file changed, 20 insertions(+)