pgsql: Use ArchiveHandle.lookaheadSize instead of a hardcoded 512

Use ArchiveHandle.lookaheadSize instead of a hardcoded 512

_discoverArchiveFormat allocates the lookahead buffer used to identify
the archive format and records its size in AH->lookaheadSize, but then
used a hardcoded size in the two plades that need it. Use the field
instead, so that the buffer and its size cannot drift apart and risk
out-of-bound reads.

Author: Aleksander Alekseev <aleksander(at)tigerdata(dot)com>
Reviewed-by: Michael Paquier <michael(at)paquier(dot)xyz>
Reviewed-by: Daniel Gustafsson <daniel(at)yesql(dot)se>
Discussion: https://postgr.es/m/CAJ7c6TPqLZOMk3hPrgLQvrOg9BCY4SV6bUC_wx54fyquRkBckw@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/8646214ee20b3754d69f9d087baa7eeb1b7e1d0b

Modified Files
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src/bin/pg_dump/pg_backup_archiver.c | 9 +++++----
1 file changed, 5 insertions(+), 4 deletions(-)