pgsql: Avoid crashing in GetOldestSnapshot() if there are no known snap

Avoid crashing in GetOldestSnapshot() if there are no known snapshots.

The sole caller expects NULL to be returned in such a case, so make
it so and document it.

Per reports from Andreas Seltenreich and Regina Obe.  This doesn't
really fix their problem, as now their RETURNING queries will say
"ERROR: no known snapshots", but in any case this function should
not dump core in a reasonably-foreseeable situation.

Report: <87vazemeda.fsf@credativ.de>
Report: <20160807051854.1427.32414@wrigleys.postgresql.org>

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/07a601eedab7c5fa4d62055fa3efacef2f38e446

Modified Files
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src/backend/utils/time/snapmgr.c | 12 +++++++-----
1 file changed, 7 insertions(+), 5 deletions(-)