Exit Dispose early if reader is already disposed

Otherwise calling Close followed by Dispose generates an exception
internally, bad for perf and for the debugging experience.

Fixes #18307

src/Microsoft.Data.Sqlite.Core/SqliteDataReader.cs

@@ -227,7 +227,7 @@ public override void Close()
         /// </param>
         protected override void Dispose(bool disposing)
         {
-            if (!disposing)
+            if (!disposing || _closed)
             {
                 return;
             }