protobuf: Fix state list growth in QProtobufDeserializerImpl::reset()

reset() appended to m_state without clearing it, so every
deserializeMessage() call left at least one stale entry behind, and
more on parse errors. A long-lived serializer, such as the one held
by a gRPC channel, grew without bound. Clear the list first, as the
JSON deserializer already does.

Pick-to: 6.12 6.8
Fixes: QTBUG-151627
Change-Id: Ia73e7208c50fbd65be9c0bd9ffe62cb235551d59
Reviewed-by: Tatiana Borisova <tatiana.borisova@qt.io>
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>