journal/catalog: fix memory leaks

journal/catalog: fix memory leaks

Various buffers were lost because finish_item() either consumed
the buffer or allocated a new one (if an entry with the same key existed).
The caller would simply forget the buffer in either case.

Also add a check for the case when a valid identifier is followed by
an empty body. We should not allow this.

Also be more consistent in error handling and always print an error
message.