Don't assume TDRC is an instance of TextIdentificationFrame (#831)
Don't assume TDRC is an instance of TextIdentificationFrame (#831)

If TDRC is encrypted, FrameFactory::createFrame() returns UnknownFrame
which causes problems in rebuildAggregateFrames() when it is assumed
that TDRC is a TextIdentificationFrame
taglib/taglib,taglib/taglib,taglib/taglib