initialize only Fiber's cfp.

fiber->cont.saved_ec.cfp should be initialized by NULL
because no vm_stack is allocated. However, cont_init()
captures current Fiber's cfp for continuation, so it should
only initialize fibers.

cont.c

@@ -1702,6 +1702,7 @@ fiber_t_alloc(VALUE fiber_value)
     fiber->cont.type = FIBER_CONTEXT;
     cont_init(&fiber->cont, th);
     fiber->cont.saved_ec.fiber_ptr = fiber;
+    fiber->cont.saved_ec.cfp = NULL;
     fiber->prev = NULL;
 
     /* fiber->status == 0 == CREATED