add fiber assertion from #1865 patch

mrbgems/mruby-fiber/src/fiber.c

@@ -3,6 +3,8 @@
 #include "mruby/class.h"
 #include "mruby/proc.h"
 
+#define fiber_ptr(o) ((struct RFiber*)mrb_ptr(o))
+
 #define FIBER_STACK_INIT_SIZE 64
 #define FIBER_CI_INIT_SIZE 8
 
@@ -62,7 +64,7 @@ static mrb_value
 fiber_init(mrb_state *mrb, mrb_value self)
 {
   static const struct mrb_context mrb_context_zero = { 0 };
-  struct RFiber *f = (struct RFiber*)mrb_ptr(self);
+  struct RFiber *f = fiber_ptr(self);
   struct mrb_context *c;
   struct RProc *p;
   mrb_callinfo *ci;
@@ -114,8 +116,9 @@ fiber_init(mrb_state *mrb, mrb_value self)
 static struct mrb_context*
 fiber_check(mrb_state *mrb, mrb_value fib)
 {
-  struct RFiber *f = (struct RFiber*)mrb_ptr(fib);
+  struct RFiber *f = fiber_ptr(fib);
 
+  mrb_assert(f->tt == MRB_TT_FIBER);
   if (!f->cxt) {
     mrb_raise(mrb, E_ARGUMENT_ERROR, "uninitialized Fiber");
   }