pgsql: Fix more portability issues with casts to Size when using off_t

Fix more portability issues with casts to Size when using off_t

This should tame the beast, as there are no other places where off_t is
used in the new error messages.

Reported again by longfin, which complained about walsender.c while I
spotted the other two ones while double-checking.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/94019c879a138454af723fa8bfc346d4a44ecd47

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