Fix use of variable before initialization

That's what you get for shadowing a variable... The "childItem" variable
on the second part of the foreach refers to one declared on the first
part, which is uninitialized.

Found by ICC 14.0:
graphicalnodeinstance.cpp(300): warning #592: variable "childItem" is used before its value is set

Change-Id: I9079a428eda90f4bb06f9ccd1adf836fee119133
Reviewed-by: Kurt Pattyn <pattyn.kurt@gmail.com>
Reviewed-by: Marco Bubke <marco.bubke@digia.com>