Handle GetOrigin() returning nullptr for Connection

[awelzel]

As discussed with @vpax externally:

A BIF with a c: connection parameter transparently gets access to the underlying/backing Connection * object of a given connection record.

zeek::detail::BifReturnVal zeek::BifFunc::set_ssl_established_bif(zeek::detail::Frame* frame, const zeek::Args* BiF_ARGS)
        {
        ...
        zeek::Connection* c = (zeek::Connection*) ((*BiF_ARGS)[0].get()->AsRecordVal()->GetOrigin());

This breaks when used with event tracing: connection records are created artificially in script land but these do do not have a backing Connection object.

Seems reasonable to check the GetOrigin() return value and triggering a runtime error and return early if it's a nullptr. Currently with event tracing, any bif trying to access the underlying Connection ends up with a null-pointer-deref crash.

We may in the future extend Zeek to be able to create a backing connection in the core for a given connection record. The check seems useful still to prevent a proliferation of null checks in individual BIFs and give a signal when something is totally off.

Today, these BIFs provide an easy way to directly crash Zeek from script land:

$ zeek -e 'global c: connection = connection($start_time=double_to_time(0.0), $duration=1sec, $history="boom", $uid="boom"); set_ssl_established(c)'
Segmentation fault (core dumped)