pgsql: Fix alignment problem with bbsink_copystream buffer.

Fix alignment problem with bbsink_copystream buffer.

bbsink_copystream wants to store a type byte just before the buffer,
but basebackup.c wants the buffer to be aligned so that it can call
PageIsNew() and PageGetLSN() on it. Therefore, instead of inserting
1 extra byte before the buffer, insert MAXIMUM_ALIGNOF extra bytes
and only use the last one.

On most machines this doesn't cause any problem (except perhaps for
performance) but some buildfarm machines with -fsanitize=alignment
dump core.

Discussion: http://postgr.es/m/CA+TgmoYx5=1A2K9JYV-9zdhyokU4KKTyNQ9q7CiXrX=YBBMWVw@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/0f47e833bf6d96c1a2c5173c8e0f6c6cc30f9592

Modified Files
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src/backend/replication/basebackup_copy.c | 18 +++++++++++-------
1 file changed, 11 insertions(+), 7 deletions(-)