The discard callback (installed via onDisconnect()) can legitimately
decide to destroy the AsyncTCP client, calling the destructors which in
turn invalidate the write mutex object. Prior to this commit, calling
_clearWriteQueue() (which takes said write mutex) risked accessing
uninitialized memory and causing memory corruption. Fixed.
This prevents a race condicion of the ::add() method modifying the queue
and being preempted by the high-priority asyncTcpSock task that in turn
flushes the same queue to the socket, therefore modifying the queue.
