pgsql: Fix inconsistent code with shared invalidations of snapshots

Fix inconsistent code with shared invalidations of snapshots

The code in charge of processing a single invalidation message has been
using since 568d413 the structure for relation mapping messages.  This
had fortunately no consequence as both locate the database ID at the
same location, but it could become a problem in the future if this area
of the code changes.

Author: Konstantin Knizhnik
Discussion: https://postgr.es/m/8044c223-4d3a-2cdb-42bf-29940840ce94@postgrespro.ru
Backpatch-through: 9.5

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/7bf20c1b8a14022c59631a2b1ce6d1e41e9494ad

Modified Files
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src/backend/utils/cache/inval.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)