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.

syntax: glob
*.pyc
*.pyv
*.swp
*.swo
tags
twistd.log
twistd.pid
bridge_constructor/generated
tests/e2e/report*
_trial_temp/
sat.egg-info
*.un~
dist
MANIFEST
build
ctags_links/
Session.vim
.build
.pytest_cache