In Curl_done() if premature is TRUE, it means this connection was said to be

DONE before the entire request operation is complete and thus we can't know in
what state it is for re-using, so we're forced to close it. In a perfect world
we can add code that keep track of if we really must close it here or not, but
currently we have no such detail knowledge.

Jerome Muffat-Meridol helped us work this out.