pgsql: Add runtime checks for bogus multixact offsets

Add runtime checks for bogus multixact offsets

It's not far-fetched that we'd try to read a multixid with an invalid
offset in case of bugs or corruption. Or if you call
pg_get_multixact_members() after a crash that left behind invalid but
unused multixids. Better to get a somewhat descriptive error message
if that happens.

Discussion: https://www.postgresql.org/message-id/3624730d-6dae-42bf-9458-76c4c965fb27@iki.fi

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/d4b7bde4183bc39c6d7a22d22188fc4aab2ba8d1

Modified Files
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src/backend/access/transam/multixact.c | 33 +++++++++++++++++++++------------
1 file changed, 21 insertions(+), 12 deletions(-)