pgsql: Fix use-after-release mistake in currtid() and currtid2() for vi

Fix use-after-release mistake in currtid() and currtid2() for views

This issue has been present since the introduction of this code as of
a3519a2 from 2002, and has been found by buildfarm member prion that
uses RELCACHE_FORCE_RELEASE via the tests introduced recently in
e786be5.

Discussion: https://postgr.es/m/20200601022055.GB4121@paquier.xyz
Backpatch-through: 9.5

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/8bc74490df252215ce5b618442900e7a107be2a1

Modified Files
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src/backend/utils/adt/tid.c | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)