glibc qsort() vulnerability

Hi hackers,

There is a bug in glibc's qsort() algorithm that runs the risk of creating
an out-of-bounds error if the comparison function is not transitive, for
example, if subtraction is used so that it can create an overflow.

See
https://packetstormsecurity.com/files/176931/glibc-qsort-Out-Of-Bounds-Read-Write.html

I checked the existing functions in the latest version of Postgres source
code and most are safe, but there were a few ones that could lead to
overflow. I do not know if these can actually lead to problems, but better
safe than sorry, so I created a patch to fix those few cases and add a
comment to one case that was not clear that it could not overflow.

Best wishes,
Mats Kindahl, Timescale