gst/mxf/: Return FALSE when parsing a metadata tag failed.

gst/mxf/: Return FALSE when parsing a metadata tag failed.

Original commit message from CVS:
* gst/mxf/mxfaes-bwf.c:
(mxf_metadata_wave_audio_essence_descriptor_handle_tag),
(mxf_metadata_aes3_audio_essence_descriptor_handle_tag):
* gst/mxf/mxfmetadata.c:
(mxf_metadata_multiple_descriptor_handle_tag):
Return FALSE when parsing a metadata tag failed.
Remove some useless return value assignments.