WebCLHelperFunctionHandler: disallow non-called functions that have no body

This refines the previous fix for #77 by allowing to declare functions
that are not called, thus allowing more code to be compiled without
introducing security problems.

The check is also now performed in WebCLFunctionCallHandler which
seems more appproriate instead of in WebCLHelperFunctionHandler.
Merge pull request #91 from eras/erkkise/#77b

WebCLHelperFunctionHandler: disallow non-called functions that have no body