alter: add an assert in _cluster replace for old tuple

_cluster on-replace trigger parses old and new tuples into
replica_def objects. One path handles the case of new_def != NULL.
The other assumes that old_def != NULL if new_def was NULL. This
is correct, because replace wouldn't happen if both tuples are
NULL. It would mean nothing changed.

Nonetheless coverity complained here that the old tuple could be
NULL even if the new one was NULL. The patch silences this warning
by adding an assert.

NO_DOC=refactor
NO_CHANGELOG=refactor
NO_TEST=not testable