varlink: cleanup use of ERRNO_IS_DISCONNECT()

varlink: cleanup 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 varlink_connect_address() which is not expected to
return any positive values, but let's be consistent anyway and move
the ERRNO_IS_DISCONNECT() invocation to the branch where
the return value is known to be negative.