Make creation of ::xo::cc more robust
In case the argument processing of
ConnectionContext create ::xo::cc -p1 ... -p2 ....
::xo::cc destroy_on_cleanup
runs into an exception leading to an ad_script_abort, the half-in
initialized object without cleanup definition will survive. At later
times, this object might be reused, containing potentially
cached results from earlier runs, which can cause troubles in error
cases.
This change takes care that errors during argument processing will
not leave half-initialized objects behind.
