[osx] check if hogPid was set before resetting it

@Memphiz I found it :)

I am not sure if its a placebo effect or not, but my build is stable now ... issues are always when switching from DTS/AC3 video to divx with mp3.

Its not always reproducible on my macmini(on MBP I can never repro it), I think its the race that we set new hogpid but its not correct. check for -1 and if true try to reset cant do any harm.