[TagLibExtractor] Fix crash on invalid Speex files
Summary:
TagLib::Ogg::Speex::File::isValid() returns true even for invalid files,
but tag() only returns a valid XiphComment when the file is valid.
Other TagLib::Ogg::* classes properly clear the valid flag when
encountering files.
See https://github.com/taglib/taglib/issues/902
BUG: 403902
Reviewers: Baloo, Frameworks, ngraham, astippich
Reviewed By: Baloo, ngraham, astippich
Subscribers: kde-frameworks-devel
Tags: Frameworks, Baloo
Differential Revision: https://phabricator.kde.org/D20913