JNI: clean up move and swap semantics in QJniObject

QJniObject is a value type with move semantics, but didn't provide swap,
so add it.
Make it safe to check a moved-from QJniObject for validity, and to
compare it with another QJniObject. As before, two invalid objects
compare as equal.

Change-Id: Ie08bb7c50da831f3e7e98e731e8ddbc0ebb3af78
Reviewed-by: Assam Boudjelthia <assam.boudjelthia@qt.io>
(cherry picked from commit 3e0c21691dd2be6911c417f4cbfaf54d10b4d634)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/kernel/qjniobject.cpp

@@ -736,6 +736,14 @@ QJniObject::QJniObject(jobject object)
 QJniObject::~QJniObject()
 {}
 
+/*!
+    \fn void QJniObject::swap(QJniObject &other)
+    \since 6.8
+
+    Swaps this object with \a other. This function is very fast and never fails.
+*/
+
+
 namespace {
 QByteArray getClassNameHelper(JNIEnv *env, const QJniObjectPrivate *d)
 {
@@ -1385,7 +1393,7 @@ bool QJniObject::isClassAvailable(const char *className)
 */
 bool QJniObject::isValid() const
 {
-    return d->m_jobject;
+    return d && d->m_jobject;
 }
 
 /*!
@@ -1411,6 +1419,8 @@ QJniObject QJniObject::fromLocalRef(jobject lref)
 
 bool QJniObject::isSameObject(jobject obj) const
 {
+    if (!d)
+        return obj == nullptr;
     if (d->m_jobject == obj)
         return true;
     if (!d->m_jobject || !obj)
@@ -1420,6 +1430,8 @@ bool QJniObject::isSameObject(jobject obj) const
 
 bool QJniObject::isSameObject(const QJniObject &other) const
 {
+    if (!other.isValid())
+        return !isValid();
     return isSameObject(other.d->m_jobject);
 }
 

src/corelib/kernel/qjniobject.h

@@ -102,6 +102,8 @@ public:
 
     ~QJniObject();
 
+    void swap(QJniObject &other) noexcept { d.swap(other.d); }
+
     template<typename Class, typename ...Args>
     static inline QJniObject construct(Args &&...args)
     {

tests/auto/corelib/kernel/qjniobject/tst_qjniobject.cpp

@@ -328,6 +328,11 @@ void tst_QJniObject::compareOperatorTests()
     QVERIFY(jstrobj != stringObject);
     QVERIFY(stringObject != jstrobj);
     QVERIFY(!invalidStringObject.isValid());
+
+    QJniObject movedTo(std::move(stringObject3));
+    QVERIFY(!stringObject3.isValid());
+    QCOMPARE_NE(movedTo, stringObject3);
+    QCOMPARE(invalidStringObject, stringObject3);
 }
 
 void tst_QJniObject::className()