[SMT-COMP] Redirect non-answers to /dev/null (#4528)

[SMT-COMP] Redirect non-answers to /dev/null (#4528)

Commit 00badd3a63a2fa568373d5c58553944b579d42bb changed our run script
to write output other than `sat`/`unsat` to a file if `$2` is passed to
it. However, it looks like StarExec does not pass that parameter to our
script despite the documentation claiming that it does [0]. This commit
makes our check more defensive by redirecting our unnecessary output to
`/dev/null` if `STAREXEC_WALLCLOCK_LIMIT` is set. I've done a quick test
run on StarExec and it looks like this does the trick.

[0]
https://wiki.uiowa.edu/display/stardev/User+Guide#UserGuide-SpecialVariables