[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