This example is exactly why RAII is the solution to this problem and not defer.
RAII is not the right solution for C. I wouldn't want C to grow constructors and destructors. So far, C only runs the code you ask it to; turning variable declaration into a hidden magic constructor call would, IMO, fly in the face of why people may choose C in the first place.
In addition, RAII has it's own complexities that need to be dealt with now, i.e. move semantics, which obviously C does not have nor will it likely ever.
In the example above, the question of "do I put defer before or after the `if err != nil` check" is deferred to the programmer. RAII forces you to handle the complexity, defer lets you shoot yourself in the foot.