The execution of the RPOPLPUSH command by the fuzzer created junk keys,
that were later being selected by RANDOMKEY and modified.
This also meant that lists were statistically tested more than other
files.
Fix the fuzzer not to pass junk key names to RPOPLPUSH, and add a check
that detects that new keys are not added by the fuzzer to detect future
similar issues.
(cherry picked from commit 3f3f678a4741e6af18230ee1862d9ced7af79faf)
aba9517542