Prevent message overrun in LocklessCommandFifo.

The previous logic in makeSpace and makeSpaceNonBlocking
was incorrect (probably a typo).  We shouldn't loop if
looping will overwrite unread messages, or if we would
make the buffer appear empty (mPut == mGet).

Change-Id: Iabc82ca94a585a7041069db97cbed7709f2d388f