If the decoder was not able to decode a frame because insufficient
information was available, e.g. missing sequence or picture header,
then allow the frame to be gracefully dropped without generating
any error.

It is also possible that a frame is not meant to be displayed but
only used as a reference, so dropping that frame is also a valid
operation since GstVideoDecoder base class has extra references to
that GstVideoCodecFrame that needs to be released.