iter: make safer by using NdbNote instead of unsafe pointers

If we have an owned note, we could lose track of the lifetime and then
crash. Let's make sure we always have an NdbNote instead

nostrdb/NdbNote.swift

@@ -34,7 +34,7 @@ struct NdbNote {
     }
 
     func tags() -> TagsSequence {
-        return .init(note: note)
+        return .init(note: self)
     }
 
     static func owned_from_json(json: String, bufsize: Int = 2 << 18) -> NdbNote? {