Non-replayable WAL records through overflows and >MaxAllocSize lengths

Hi,

Xlogreader limits the size of what it considers valid xlog records to
MaxAllocSize; but this is not currently enforced in the
XLogRecAssemble API. This means it is possible to assemble a record
that postgresql cannot replay.
Similarly; it is possible to repeatedly call XlogRegisterData() so as
to overflow rec->xl_tot_len; resulting in out-of-bounds reads and
writes while processing record data;

PFA a patch that attempts to fix both of these issues in the insertion
API; by checking against overflows and other incorrectly large values
in the relevant functions in xloginsert.c. In this patch, I've also
added a comment to the XLogRecord spec to document that xl_tot_len
should not be larger than 1GB - 1B; and why that limit exists.

Kind regards,

Matthias van de Meent