Set disposed marker line to -1

Another indicator that it's disposed/invalid

src/common/buffer/Marker.ts

@@ -29,6 +29,7 @@ export class Marker extends Disposable implements IMarker {
       return;
     }
     this.isDisposed = true;
+    this.line = -1;
     // Emit before super.dispose such that dispose listeners get a change to react
     this._onDispose.fire();
   }