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
------
REL9_3_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/24aa77ec9549f2ca67220ef7b5d7f2dce5863d31

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