Comment 1 for bug 1468495

It needs with-pinned-objects around the call to (setf (context-register context callee-register-offset) (get-lisp-obj-address new-callee)), since %set-context-register is a function and get-lisp-obj-address passes a fixnum representation of the function, which doesn't hold it. context is registered by fake_foreign_function_call, so once it's there, it's pinned down.
Will commit after the freeze.