QReadWriteLock: initialize member var currentWriter

Found by valgrind's memcheck.

Change-Id: I68f7c9279033e76d82f511b9d095548b4ef45723
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>

src/corelib/thread/qreadwritelock_p.h

@@ -65,7 +65,7 @@ class QReadWriteLockPrivate
 public:
     QReadWriteLockPrivate(bool isRecursive = false)
         : readerCount(0), writerCount(0), waitingReaders(0), waitingWriters(0),
-        recursive(isRecursive), id(0) {}
+        recursive(isRecursive), id(0), currentWriter(nullptr) {}
 
     QMutex mutex;
     QWaitCondition writerCond;