varlink: add helper that clears the currently processed incoming message JSON object

varlink: add helper that clears the currently processed incoming message JSON object

Some minor refactoring. This adds a helper call whose only job is to
unref the JSON object of the currently processed incoming message.

This doesn't make too much sense on its own, given this just replaces
one line by another. However, in a later patch when we'll add fd passing
we'll extend the function to also destroy associated fds, and then it
will start to make more sense.