Fix for bug reported on bf-committers: RNA function nodetree.links.new does not remove existing links when connecting to a linked input. The check for socket connection limits was 1 off, needs to add 1 to the link counter to anticipate the newly added link.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@55436 954f8c5b-7b00-dc11-b283-0030488c597c