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