CVE-2026-44328: free5GC's SMF UPI DELETE /upi/v1/upNodesLinks/{ref} panics on AN-node deletion via nil UPF dereference; unauthenticated, state-mutating

free5GC’s SMF mounts the UPI management route group without inbound OAuth2 middleware (same root cause as the broader UPI auth gap reported in free5gc/free5gc#887). On top of that, the DELETE /upi/v1/upNodesLinks/{upNodeRef} handler unconditionally dereferences upNode.UPF after the type-guarded async release, even though AN-typed nodes are constructed without a UPF object. As a result, a single unauthenticated DELETE /upi/v1/upNodesLinks/gNB1 request crashes the handler with a nil-pointer panic AND mutates the in-memory user-plane topology before panicking (the UpNodeDelete(upNodeRef) line runs first). This is an unauthenticated, state-mutating panic-DoS sink that an off-path network attacker can trigger by name against any AN entry.