pgsql: Fix variable initialization when using buffering build with GiST

Fix variable initialization when using buffering build with GiST

This can cause valgrind to complain, as the flag marking a buffer as a
temporary copy was not getting initialized.

While on it, fill in with zeros newly-created buffer pages. This does
not matter when loading a block from a temporary file, but it makes the
push of an index tuple into a new buffer page safer.

This has been introduced by 1d27dcf, so backpatch all the way down to
9.4.

Author: Alexander Lakhin
Discussion: https://postgr.es/m/15899-0d24fb273b3dd90c@postgresql.org
Backpatch-through: 9.4

Branch
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REL9_5_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/b9e1538fcf35708f19ce3f86c22b9bd91a0d2820

Modified Files
--------------
src/backend/access/gist/gistbuildbuffers.c | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)