pgsql: Fix memory leak on DSM slot exhaustion.

Fix memory leak on DSM slot exhaustion.

If we attempt to create a DSM segment when no slots are available,
we should return the memory to the operating system. Previously
we did that if the DSM_CREATE_NULL_IF_MAXSEGMENTS flag was
passed in, but we didn't do it if an error was raised. Repair.

Back-patch to 9.4, where DSM segments arrived.

Author: Thomas Munro
Reviewed-by: Robert Haas
Reported-by: Julian Backes
Discussion: https://postgr.es/m/CA%2BhUKGKAAoEw-R4om0d2YM4eqT1eGEi6%3DQot-3ceDR-SLiWVDw%40mail.gmail.com

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/93be45245272fc9f59caecc3db949b49b464a2a1

Modified Files
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src/backend/storage/ipc/dsm.c | 17 ++++++++---------
1 file changed, 8 insertions(+), 9 deletions(-)