Before 565043fae7f the return 0 was not there, which made the code return a freed struct instead. That change was made over 14 years ago...
I think it might rather be 12d9d41e83a1137846ae60ef79738e10e90825d0. I have never managed to run into Shuffler really, but one solution probably just is to close mystdin if stdin_str is the empty string?