pgsql: Add additional sanity checks when reading a blkreftable.

Add additional sanity checks when reading a blkreftable.

Code elsewhere in the system assumes that fork numbers and chunk sizes
are within bounds, so the code that reads those quantities from disk
should validate that they are. Without these additional checks, a
corrupted file can cause us to index off the end of fork number or chunk
entry arrays, potentially resulting in a crash.

Reported-by: oxsignal <awo(at)kakao(dot)com> (chunk sizes)
Reported-by: Robert Haas <rhaas(at)postgresql(dot)org> (fork numbers)
Reviewed-by: Daniel Gustafsson <daniel(at)yesql(dot)se>
Discussion: http://postgr.es/m/CA+TgmoYP8RKoBGosS7C6Fdr-GNCfyz_W1zmK=Tx1Fe0ZvzGh0g@mail.gmail.com
Backpatch-through: 17

Branch
------
REL_18_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/bcc428a23a69ee6f3bcc2ce20655da68eed0e7aa

Modified Files
--------------
src/common/blkreftable.c | 22 ++++++++++++++++++++++
1 file changed, 22 insertions(+)