pgsql: Fix unsafe reference into relcache in constructed CommentStmt.

Fix unsafe reference into relcache in constructed CommentStmt.

The CommentStmt made by RebuildConstraintComment() has to pstrdup the
relation name, else it will contain a dangling pointer after that
relcache entry is flushed. (I'm less sure that pstrdup'ing conname
is necessary, but let's be safe.) Failure to do this leads to weird
errors or crashes, as reported by Marko Elezovic.

Bug introduced by commit e42375fc8, so back-patch to 9.5 as that was.

Fix by David Rowley, regression test by Michael Paquier

Discussion: https://postgr.es/m/DB6PR03MB30775D58E732D4EB0C13725B9AE00@DB6PR03MB3077.eurprd03.prod.outlook.com

Branch
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REL9_5_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/53a1aa9f9a6f3711ff5766beca135ddce3f3b4ad

Modified Files
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src/backend/commands/tablecmds.c | 4 ++--
src/test/regress/expected/alter_table.out | 33 +++++++++++++++++++++++++++++++
src/test/regress/sql/alter_table.sql | 18 +++++++++++++++++
3 files changed, 53 insertions(+), 2 deletions(-)