Avoid direct return from ecall(); fix #3441

There's incompatibility left for mruby.

When you return from `ensure` clause, mruby simply ignores the return
value.  CRuby returns from the method squashing the exception raised.

```
def f
  no_such_method() # NoMethodError
ensure
  return 22
end

p f() # CRuby prints `22`
```

src/vm.c

@@ -1652,7 +1652,7 @@ RETRY_TRY_BLOCK:
         switch (GETARG_B(i)) {
         case OP_R_RETURN:
           /* Fall through to OP_R_NORMAL otherwise */
-          if (proc->env && !MRB_PROC_STRICT_P(proc)) {
+          if (ci->acc >=0 && proc->env && !MRB_PROC_STRICT_P(proc)) {
             struct REnv *e = top_env(mrb, proc);
 
             if (!MRB_ENV_STACK_SHARED_P(e)) {