kin-openapi spec loader panics on a self-referential additionalProperties $ref (nil-pointer DoS)

The defect ---------- Confirmed at HEAD f0407d5, v0.144.0. openapi3/loader.go, inside resolveComponent's drill closure around line 455, the case SchemaRef branch for pathPart == "additionalProperties" dereferences c.Value.AdditionalProperties with no nil-check on c.Value: case SchemaRef: if...