Fix memory leak when unregistering a gesture recognizer

When a gesture was unrecognized, then it would add itself to the
obsolete gestures hash. This was cleaned up only on application exit,
but as the unregister call happens whenever a widget that had registered
gestures was deleted then the hash could grow quite considerably.

In order to ensure the original intention of the code here, we only
call unregisterGestureRecognizer() when there is a QGestureManager in
place to call it on. Otherwise it would create a memory leak in itself.

Change-Id: I2342f3f737b28be4af7ed531d83f02197eb66c0e
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>

src/widgets/kernel/qgesturemanager.cpp

@@ -143,11 +143,6 @@ Qt::GestureType QGestureManager::registerGestureRecognizer(QGestureRecognizer *r
 void QGestureManager::unregisterGestureRecognizer(Qt::GestureType type)
 {
     QList<QGestureRecognizer *> list = m_recognizers.values(type);
-    while (QGestureRecognizer *recognizer = m_recognizers.take(type)) {
-        // ensuring an entry exists causes the recognizer to be deleted on destruction of the manager
-        auto &gestures = m_obsoleteGestures[recognizer];
-        Q_UNUSED(gestures);
-    }
     foreach (QGesture *g, m_gestureToRecognizer.keys()) {
         QGestureRecognizer *recognizer = m_gestureToRecognizer.value(g);
         if (list.contains(recognizer)) {

src/widgets/kernel/qgesturerecognizer.cpp

@@ -41,6 +41,7 @@
 
 #include "private/qgesture_p.h"
 #include "private/qgesturemanager_p.h"
+#include "private/qapplication_p.h"
 
 #ifndef QT_NO_GESTURES
 
@@ -231,6 +232,11 @@ Qt::GestureType QGestureRecognizer::registerRecognizer(QGestureRecognizer *recog
 */
 void QGestureRecognizer::unregisterRecognizer(Qt::GestureType type)
 {
+    auto qAppPriv = QApplicationPrivate::instance();
+    if (!qAppPriv)
+        return;
+    if (!qAppPriv->gestureManager)
+        return;
     QGestureManager::instance()->unregisterGestureRecognizer(type);
 }