pgsql: Fix two issues in TOAST decompression.

Fix two issues in TOAST decompression.

pglz_maximum_compressed_size() potentially underestimated the amount
of compressed data required to produce N bytes of decompressed data;
this is a fault in commit 11a078cf8.

Separately from that, pglz_decompress() failed to protect itself
against corrupt compressed data, particularly off == 0 in a match
tag. Commit c60e520f6 turned such a situation into an infinite loop,
where before it'd just have resulted in garbage output.

The combination of these two bugs seems like it may explain bug #16694
from Tom Vijlbrief, though it's impossible to be quite sure without
direct inspection of the failing session. (One needs to assume that
the pglz_maximum_compressed_size() bug caused us to fail to fetch the
second byte of a match tag, and what happened to be there instead was
a zero. The reported infinite loop is hard to explain without off == 0,
though.)

Aside from fixing the bugs, rewrite associated comments for more
clarity.

Back-patch to v13 where both these commits landed.

Discussion: https://postgr.es/m/16694-f107871e499ec114@postgresql.org

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/dfc797730fc7a07c0e6bd636ad1a564aecab3161

Modified Files
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src/common/pg_lzcompress.c | 101 +++++++++++++++++++++++++++++----------------
1 file changed, 66 insertions(+), 35 deletions(-)