Remove race possibilities from slip mode

I think this was causing https://bugs.launchpad.net/mixxx/+bug/1362394, and I know for sure there were races in this code, so even if it doesn't fix that bug it's an improvement.
I did not fully understand this fix. But if you wish this function to be interruptible, you need to use something like: 
int slipStatus = m_SlipStatus.fetchAndStoreRelease(maskval);
at the beginning of this function.