Fix explicit valgrind interaction in read_stream.c.
This is a back-patch of commits 2a8a0067 and 2509b857 into
REL_17_STABLE. It's doesn't fix any known live bug in PostgreSQL v17
itself, but an extension could in theory have used the per-buffer data
feature and seen spurious errors under Valgrin
Fix explicit valgrind interaction in read_stream.c.
By calling wipe_mem() on per-buffer data memory that has been released,
we are also telling Valgrind that the memory is "noaccess". We need to
set it to "undefined" before giving it to the registered callback to
fill in, when a slot is reused.
