Clear the cached value when re-using the JSON deserializer

Ensure that we clear the deserializer state when re-use it after
unsuccessful deresializing.

Pick-to: 6.9 6.8
Change-Id: I6e0c489706b90e8daaa49c1872642a7cbefee2e3
Reviewed-by: Dennis Oberst <dennis.oberst@qt.io>

src/protobuf/qprotobufjsonserializer.cpp

@@ -258,6 +258,7 @@ QProtobufJsonDeserializerImpl::~QProtobufJsonDeserializerImpl()
 
 void QProtobufJsonDeserializerImpl::reset(QJsonObject obj)
 {
+    clearCachedValue();
     m_state.clear();
     if (!obj.isEmpty())
         m_state.push_back({ obj });