pgsql: Fix DetermineSafeOldestOffset for the case where there are no mx

Fix DetermineSafeOldestOffset for the case where there are no mxacts.

Commit b69bf30b9bfacafc733a9ba77c9587cf54d06c0c failed to take into
account the possibility that there might be no multixacts in existence
at all.

Report by Thomas Munro; patch by me.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/312747c224be4c0a880b5172a010e980f8b654ca

Modified Files
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src/backend/access/transam/multixact.c |   23 +++++++++++++++++------
1 file changed, 17 insertions(+), 6 deletions(-)