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_5_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/8fcc2fb98064adfe6b14f6ac6964c59f5ace09cd

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