Scripted mobility for nodes not on the current canvas causes a gui error

```
What steps will reproduce the problem?
1. load sample1.imn
2. create a new canvas
3. select nodes n5, n6, n7, n8, n9, wlan10 and move them to Canvas2.
4. start the session

What is the expected output? What do you see instead?

An Application Error occurs:

missing operand at _@_
in expression "(672.0 - _@_)"
missing operand at _@_
in expression "(672.0 - _@_)"
(parsing expression "(672.0 - )")
invoked from within
"expr ($xpos - $x)"
(procedure "moveNodeAbs" line 14)
invoked from within
"moveNodeAbs $c $node [expr {$zoom * $vals(xpos)}] [expr {$zoom *
$vals(ypos)}]"
(procedure "apiNodeModify" line 12)
invoked from within
"apiNodeModify $node vals "
(procedure "parseNodeMessage" line 149)
invoked from within
"parseNodeMessage $bytes $len $flags "
(procedure "receiveMessage" line 49)
invoked from within
"receiveMessage sock1afe000"
```

Original issue reported on code.google.com by `tgo...@gmail.com` on 30 Jan 2015 at 10:57