Citadel Holding's offer to top any rival takeover bid for Valley Federal Savings & Loan was extended until Feb. 12. Glendale-based Citadel, parent of Fidelity Federal Savings & Loan, offered Jan. 14 to buy Valley Federal for $18.50 a share in cash, or $107.3 million. Citadel also offered to pay $1 a share more than any competing cash offer from another S&L; that might emerge. No such offer had surfaced before Citadel's original deadline Tuesday. Valley Federal itself has yet to respond to Citadel's proposal.