Fix CI misalignment between Fiber#resume and Fiber.yield

When a fiber switched by `Fiber.yield` is resumed by `Fiber#resume` by C, it is necessary to pop CI with `fiber_switch()`.
Previously, CI misalignment caused inconsistencies, including crashes, on the next `Fiber#resume`.

src/vm.c

@@ -1839,6 +1839,7 @@ RETRY_TRY_BLOCK:
           }
           recv = MRB_METHOD_FUNC(m)(mrb, recv);
         }
+        mrb_assert(mrb->c->ci > mrb->c->cibase);
         mrb_gc_arena_shrink(mrb, ai);
         if (mrb->exc) goto L_RAISE;
         ci = mrb->c->ci;