Sometimes we get an EAGAIN error in the due to the socket being
nonblocking. In the setup phase we just allocated the new buffer
and this causes a leak. Free it in this case.
The header returned from parse_header contains references
to the buffer it was used to parse from, and in some
cases we dereference these headers after freeing the buffer.
For instance this happens when we're filtering a message, and
then we later look at the destination to figure out what
kind of error to send back.
I couldn't find any cases where this would let the client
do anything other than return a different error value, but
this is still possibly a security issue.
Make sure we can only send message calls (not e.g. unicast signals)
to the bus. Also, whitelist the list of allowed interfaces to
org.freedesktop.DBus and org.freedesktop.DBus.Introspectable
See https://github.com/flatpak/flatpak/issues/343 for the origin
discussion of this.
