fix: prevents nil dereference on UP node removal

[DBGR18]

fix issue#905

This pull request makes a targeted improvement to the DeleteUpNodeLink handler to ensure that the CancelAssociation method is only called on valid UPF nodes. Specifically, it adds a check to confirm that the UPF field is not nil before invoking CancelAssociation, and moves the call to occur only when the node type is UPNODE_UPF and a valid UPF exists.

Node deletion logic improvements:

• Updated the conditional in DeleteUpNodeLink to call CancelAssociation only if upNode.Type is UPNODE_UPF and upNode.UPF is not nil, preventing potential nil pointer dereference errors.
• Removed redundant call to upNode.UPF.CancelAssociation() outside the conditional, ensuring the method is only called when appropriate.

[Copilot]

Pull request overview

This PR addresses a panic scenario in the SMF UPI SBI handler by ensuring CancelAssociation() is only invoked for valid UPF nodes (and only when the UPF pointer is non-nil), aligning the deletion flow with the intended UP-node types.

Changes:

• Guard CancelAssociation() behind upNode.Type == UPNODE_UPF && upNode.UPF != nil.
• Move CancelAssociation() into the UPF-specific branch and remove the unconditional call after UpNodeDelete.
• Keep UPF resource release triggered only for UPF nodes.

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[d11nn]

@Alonza0314 LGTM!