pgsql: Fix use-after-free issue in slot synchronization.

Fix use-after-free issue in slot synchronization.

Author: Shlok Kyal <shlok.kyal.oss@gmail.com>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
Backpatch-through: 18, where it was introduced
Discussion: https://postgr.es/m/CANhcyEXMrcEdzj-RNGJam0nJHM4y+ttdWsgUCFmXciM7BNKc7A@mail.gmail.com

Branch
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REL_18_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/8df3d355db0fcb56535644c5ea2f4ae6f7dfacdf

Modified Files
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src/backend/replication/logical/slotsync.c | 3 ++-
src/backend/replication/slotfuncs.c        | 3 ++-
2 files changed, 4 insertions(+), 2 deletions(-)