Blender Git Loki

Nodes: Fix drag link from output to already linked Multi-Input Socket

This patch fixes a visual bug related to connecting an output socket to
a Multi-Input Socket, that already has a link to that same output.
In this case, the drag link got a new index and snapped to a new
position. This path makes the drag link snap to the same position as the
first link between the two sockets.

Differential Revision: https://developer.blender.org/D10689