Fix #428

Add binding check for generate verb in loops. The check is present
in other loop verbs.

test/suite8.janet

@@ -319,4 +319,10 @@ neldb\0\0\0\xD8\x05printG\x01\0\xDE\xDE\xDE'\x03\0marshal_tes/\x02
 (check-jdn "a string")
 (check-jdn @"a buffer")
 
+# Issue 428
+(var result nil)
+(defn f [] (yield {:a :ok}))
+(assert-no-error "issue 428 1" (loop [{:a x} :generate (fiber/new f)] (set result x)))
+(assert (= result :ok) "issue 428 2")
+
 (end-suite)