The returned list contains full references, so make sure we unref them
before going on. Note that it's ok to return a pointer to one object
inside this list even if we're unref-ing them all, because we're sure
that the caller knows it's peek-ing a port object.
The return status of mm_base_modem_at_command_finish() already
specifies whether an error has happened or not, so skip creating the
GError if we don't care about the actual error details.