win64 r55714 builder.blender.org

--- Short description of error ---
When a Viewer Node is grouped inside a Node Group the output of the node corrupts all Viewer Nodes in the tree
Split Viewer nodes, as well as Viewer Nodes which are not even connected to the Node Group are affected.


--- Steps for others to reproduce the error (preferably based on attached .blend file) ---
Check the attached file.

The NodeGroup is just a Blur node with a Viewer Node. It simply blurs the image.

1.Press F12 to render the image. Press Esc to go back to the Node Editor.
2.Change the Factor of the Split View node. The backdrop should show an incorrect result.
3.Click on the Viewer Node connected to the Render Layer. Even this viewer shows the output of the NodeGroup.

Hot to fix:
===========
1. Open the NodeGroup and delete the Viewer Node. All Nodes should now work as expected.

Thanks.