cabinet: Fix NULL pointer dereference (Coverity)

[Andrew Talbot]

> This last check is pointless.  If perf is NULL, we bail out and return
> NULL a few lines up.  Also, please add a test showing that native does
> not crash if a NULL perf is sent in.  It's weird, but we need to be
> consistent with native, as some apps depend on such a crash.
> 

Whoops! Thanks, James; I was a bit lax, there.

-- Andy.