typefind/xdgmime: Validate mimetypes to be valid GstStructure names before using them
On macOS, for example, "text/*" can be returned as mimetype for plaintext files but we don't allow '*' in structure names and this would cause critical warnings.
It's a valid mimetype but not a valid structure name.
Fixes gstreamer#616 (closed)
Edited by Sebastian Dröge