mp4: Simplify parse_tag().

mp4: Simplify parse_tag().

We don't care about arbitrarily named tags, and those tags we're
interested in are generally present in form of the standard tags
(ATOM_TITLE, ATOM_ARTIST etc.).

Since we now always call get_metadata_name() to get the string
representation of the tag, we don't need to make a copy any more,
just pass the const pointer directly to tag_add_field().

With this change in place it is obvious that we never pass a NULL or
empty tag name to tag_add_field(), and we don't pass a NULL pointer
for the value argument either, so remove the safety check.