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
```
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