cups: fix CVE-2022-26691

In scheduler/cert.c the previous algorithm didn't expect the strings can
have a different length, so one string can be a substring of the other
and such substring was reported as equal to the longer string.

Backport patch from upstream to fix:
https://github.com/OpenPrinting/cups/commit/de4f8c196106033e4c372dce3e91b9d42b0b9444



CVE: CVE-2022-26691

Signed-off-by: Steve Sakoman <steve@sakoman.com>