core: don't callback `self.conn_deferred` if already called: if the connection has been lost (e.g. forced disconnection by server), `self.conn_deferred` may have already been called, thus this is now checked to avoid an exception.

FROM libervia/prosody:e2e

LABEL maintainer="Goffi <tmp_dockerfiles@goffi.org>"

RUN prosodyctl register demo server1.test demo