pgsql: In recordExtensionInitPriv(), keep the scan til we're done with

In recordExtensionInitPriv(), keep the scan til we're done with it

For reasons of sheer brain fade, we (I) was calling systable_endscan()
immediately after systable_getnext() and expecting the tuple returned
by systable_getnext() to still be valid.

That's clearly wrong.  Move the systable_endscan() down below the tuple
usage.

Discovered initially by Pavel Stehule and then also by Alvaro.

Add a regression test based on Alvaro's testing.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/99f2f3c19ae7d6aa2950a9bdb549217c5a60d941

Modified Files
--------------
src/backend/catalog/aclchk.c                                  | 4 ++--
src/test/modules/test_extensions/Makefile                     | 6 +++---
src/test/modules/test_extensions/expected/test_extensions.out | 3 +++
src/test/modules/test_extensions/sql/test_extensions.sql      | 4 ++++
src/test/modules/test_extensions/test_ext6--1.0.sql           | 1 +
src/test/modules/test_extensions/test_ext6.control            | 5 +++++
6 files changed, 18 insertions(+), 5 deletions(-)