pgsql: Check for relation length overrun soon enough.

Check for relation length overrun soon enough.

We don't allow relations to exceed 2^32-1 blocks, because block
numbers are 32 bits and the last possible block number is reserved
to mean InvalidBlockNumber. There is a check for this in mdextend,
but that's really way too late, because the smgr API requires us to
create a buffer for the block-to-be-added, and we do not want to
have any buffer with blocknum InvalidBlockNumber. (Such a case
can trigger assertions in bufmgr.c, plus I think it might confuse
ReadBuffer's logic for data-past-EOF later on.) So put the check
into ReadBuffer.

Per report from Christoph Berg. It's been like this forever,
so back-patch to all supported branches.

Discussion: https://postgr.es/m/YTn1iTkUYBZfcODk@msg.credativ.de

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/cc4de2bba02ffcd11caf980fb4c1a500f64bf137

Modified Files
--------------
src/backend/storage/buffer/bufmgr.c | 9 +++++++++
src/backend/storage/smgr/md.c | 3 ++-
2 files changed, 11 insertions(+), 1 deletion(-)