pgsql: Fix access-off-end-of-array in clog.c.

Fix access-off-end-of-array in clog.c.

Sloppy loop coding in set_status_by_pages() resulted in fetching one array
element more than it should from the subxids[] array. The odds of this
resulting in SIGSEGV are pretty small, but we've certainly seen that happen
with similar mistakes elsewhere. While at it, we can get rid of an extra
TransactionIdToPage() calculation per loop.

Per report from David Binderman. Back-patch to all supported branches,
since this code is quite old.

Discussion: https://postgr.es/m/HE1PR0802MB2331CBA919CBFFF0C465EB429C710@HE1PR0802MB2331.eurprd08.prod.outlook.com

Branch
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REL9_2_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/6d2ef1cb99ebaa334c9f0406be142635d02c53d0

Modified Files
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src/backend/access/transam/clog.c | 12 +++++++++---
1 file changed, 9 insertions(+), 3 deletions(-)