Must the mrb_sys_fail() function raise an exception

Normal return of control from the `mrb_sys_fail()` function has unexpected results.
Example: double free, malloc with huge size used as error integer

The following is an example of an actual crash:

```console
% cat crash.rb
def SystemCallError._sys_fail(*a)
  nil
end

File.readlink("/404")

% bin/mruby crash.rb
zsh: segmentation fault (core dumped)  bin/mruby crash.rb
```

src/error.c

@@ -521,7 +521,7 @@ exception_call:
   return mesg;
 }
 
-MRB_API void
+MRB_API mrb_noreturn void
 mrb_sys_fail(mrb_state *mrb, const char *mesg)
 {
   struct RClass *sce;
@@ -537,9 +537,8 @@ mrb_sys_fail(mrb_state *mrb, const char *mesg)
       mrb_funcall_id(mrb, mrb_obj_value(sce), MRB_SYM(_sys_fail), 1, mrb_fixnum_value(no));
     }
   }
-  else {
-    mrb_raise(mrb, E_RUNTIME_ERROR, mesg);
-  }
+
+  mrb_raise(mrb, E_RUNTIME_ERROR, mesg);
 }
 
 MRB_API mrb_noreturn void