tst_QByteArray: check replace() doesn't replace the terminating \0

It doesn't.

Pick-to: 6.5 5.15
Change-Id: Ica3fb2b8a65d0f9d14490ecdcce72eba82258916
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
Reviewed-by: Ahmad Samir <a.samirh78@gmail.com>
(cherry picked from commit fff69288781bf946ad645fd781ad51ab48ebcbdc)
(cherry picked from commit 8e2c6a590429892d9fecf9a0ccc7c6a00d6c7b6d)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

tests/auto/corelib/text/qbytearray/tst_qbytearray.cpp

@@ -76,6 +76,7 @@ private slots:
     void replaceWithSpecifiedLength();
     void replaceWithEmptyNeedleInsertsBeforeEachChar_data();
     void replaceWithEmptyNeedleInsertsBeforeEachChar();
+    void replaceDoesNotReplaceTheTerminatingNull();
 
     void number();
     void number_double_data();
@@ -1601,6 +1602,23 @@ void tst_QByteArray::replaceWithEmptyNeedleInsertsBeforeEachChar()
     }
 }
 
+void tst_QByteArray::replaceDoesNotReplaceTheTerminatingNull()
+{
+    // Try really hard to replace the implicit terminating '\0' byte:
+    constexpr char content[] = "Hello, World!";
+#define CHECK(...) do { \
+        QByteArray ba(content); \
+        QCOMPARE(std::as_const(ba).data()[ba.size()], '\0'); \
+        ba.replace(__VA_ARGS__); \
+        QCOMPARE(ba, content); \
+        QCOMPARE(std::as_const(ba).data()[ba.size()], '\0'); \
+    } while (false)
+    CHECK('\0', 'a');
+    CHECK(QByteArrayView{"!", 2}, // including \0, matches end of `ba`
+          QByteArrayView{"!!"});
+#undef CHECK
+}
+
 void tst_QByteArray::number()
 {
     QCOMPARE(QByteArray::number(quint64(0)), QByteArray("0"));