fix out-of-bounds access on trailing percent sign in tr() argument

tr() recognizes %n and %Ln. it offers no way to escape lone percent
signs, which implies that they must be interpreted verbatim, which is
what the code actually does. except that it would run off the end if the
% appeared at the end of the string.

Fixes: QTBUG-57171
Done-with: Mateusz Starzycki <mstarzycki@gmail.com>
Change-Id: Icf81925c482be1ea66ec8daafb3e92ad17ea7fab
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@gmx.de>

src/corelib/kernel/qcoreapplication.cpp

@@ -2097,9 +2097,13 @@ static void replacePercentN(QString *result, int n)
         int len = 0;
         while ((percentPos = result->indexOf(QLatin1Char('%'), percentPos + len)) != -1) {
             len = 1;
+            if (percentPos + len == result->length())
+                break;
             QString fmt;
             if (result->at(percentPos + len) == QLatin1Char('L')) {
                 ++len;
+                if (percentPos + len == result->length())
+                    break;
                 fmt = QLatin1String("%L1");
             } else {
                 fmt = QLatin1String("%1");

tests/auto/corelib/kernel/qcoreapplication/tst_qcoreapplication.cpp

@@ -928,6 +928,12 @@ void tst_QCoreApplication::threadedEventDelivery()
     thread.start();
     QVERIFY(thread.wait(1000));
     QCOMPARE(receiver.recordedEvents.contains(QEvent::User + 1), eventsReceived);
+
+}
+
+void tst_QCoreApplication::testTrWithPercantegeAtTheEnd()
+{
+    QCoreApplication::translate("testcontext", "this will crash%", "testdisamb", 3);
 }
 
 #if QT_CONFIG(library)

tests/auto/corelib/kernel/qcoreapplication/tst_qcoreapplication.h

@@ -59,6 +59,7 @@ private slots:
     void applicationEventFilters_auxThread();
     void threadedEventDelivery_data();
     void threadedEventDelivery();
+    void testTrWithPercantegeAtTheEnd();
 #if QT_CONFIG(library)
     void addRemoveLibPaths();
 #endif