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

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