Set the queue handle to null before stop & destroy. (#525)

g_sendQueueHandle is set to NULL to prevent new requests from being enqueued.
It avoids a race condition which could occur when adding elements to QueueingThread if it's already stopped.

Race condition scenario:-
Background:-
-> If a thread has called CAQueueingThreadStop(), it signals the condition variable
to wake up the CAQueueingThreadBaseRoutine.

-> After signaling, it waits on the same condition variable
to wait for the completion of CAQueueingThreadBaseRoutine.

-> CAQueueingThreadBaseRoutine will finish its current task and signals the condition variable.

-> CAQueueingThreadStop should wake up upon signal and return back to caller.

Issue:-
-> CAQueueingThreadAddData also signals the condition variable which can wake up the thread that called CAQueueingThreadStop.
-> CAQueueingThreadStop returns back to caller assuming that the CAQueueingThreadBaseRoutine has stopped. But it could still be running.

https://github.sec.samsung.net/RS7-IOTIVITY/IoTivity/pull/525
(cherry picked from eb98e19b49ab4d4475449f99f3a4a53607dbea01)

Change-Id: I5b22c034b30b00fc1ae7f08c37c278c29910345c
Signed-off-by: Senthil Kumar G S <senthil.gs@samsung.com>
Signed-off-by: DoHyun Pyun <dh79.pyun@samsung.com>