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.
using m_SlipStatus.fetchAndAddAcquire(0) instead of just m_SlipStatus makes no difference. 
fetchAndAddAcquire(0) is an atomic operation to add a value to a memory address an return the original value. 
Just reading an int is always atomic (operator int())  

```
inline int QBasicAtomicInt::fetchAndAddAcquire(int valueToAdd)
{
    return fetchAndAddOrdered(valueToAdd);
}

inline int QBasicAtomicInt::fetchAndAddOrdered(int valueToAdd)
{
    asm volatile("lock\n"
                 "xaddl %0,%1"
                 : "=r" (valueToAdd), "+m" (_q_value)
                 : "0" (valueToAdd)
                 : "memory");
    return valueToAdd;
}

    inline operator int() const
    {
        return _q_value;
    }
```