sd-bus: fix use of ERRNO_IS_DISCONNECT()

sd-bus: fix use of ERRNO_IS_DISCONNECT()

Given that ERRNO_IS_DISCONNECT() also matches positive values,
make sure this macro is not called with arguments that do not have
errno semantics.

In this case the argument passed to ERRNO_IS_DISCONNECT() is the value
returned by bus_socket_process_watch_bind(), bus_socket_process_opening(),
and bus_socket_process_authenticating() which can legitimately return
positive values without errno semantics, so fix this by moving the
ERRNO_IS_DISCONNECT() invocation to the branch where the return value
is known to be negative.