QRingBuffer: simplify QRingChunk::clear()

Clear should bring the QRingChunk into the default-constructe state,
so just assign {} instead of calling assign(QByteArray()), which is
equivalent, but has to manipulate QBA's atomic ref-count, which makes
it unlikely the compiler will optimize this call.

Change-Id: Idab9c92ec84392d484395042db2427c668756efa
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
(cherry picked from commit 5f305e51b5058b0101c76210b2d804a4a4867362)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/tools/qringbuffer_p.h

@@ -167,7 +167,7 @@ public:
     }
     inline void clear()
     {
-        assign(QByteArray());
+        *this = {};
     }
 
 private: