ffi/unsafe/objc: check for bad protocol

Raise an exception instead of installing NULL as a protocol for a
class.

racket/collects/ffi/unsafe/objc.rkt

@@ -697,6 +697,8 @@
       (objc_addClass (cast id _Class _objc_class-pointer))))
 
 (define (add-protocol id proto)
+  (unless proto
+    (error 'add-protocol "NULL protocol"))
   (if class_addProtocol
       (class_addProtocol id proto)
       (add-protocol-the-hard-way id proto)))