RunTimeTyped : IECore::assertedStaticCast allow null source pointer

* use approach taken by boost polymorphic_downcast and compare source
  pointer to casted pointer, this has added benefit of not compiling
  if Target type cannot be converted to Source type so should detect
  programming errors at compile time.

Author: proberts-cinesite

include/IECore/RunTimeTyped.inl

@@ -71,14 +71,14 @@ T *runTimeCast( S *src )
 template<typename T, typename S>
 boost::intrusive_ptr<T> assertedStaticCast( const boost::intrusive_ptr<S> &src )
 {
-	assert( runTimeCast<T>( src ) );
+	assert( runTimeCast<T>( src ) == src ); // NOTE : see boost polymorphic_downcast
 	return boost::static_pointer_cast<T>( src );
 }
 
 template<typename T, typename S>
 T* assertedStaticCast( S* src )
 {
-	assert( runTimeCast<T>( src ) );
+	assert( runTimeCast<T>( src ) == src ); // NOTE : see boost polymorphic_downcast
 	return static_cast<T *>( src );
 }