JNI crashes when trying to load invalid file

[pjreiniger]

The C code segfaults if the FILE* fp pointer is null. I think the best thing to do would be having the read/write file classes do some defensive coding and check for null first. I was going to make a PR for that, but I saw #59, which is similar in concept, just higher up the chain.

It is a pretty cryptic crash that most students will probably not be able to figure out quickly, so I think it is a valuable update to make.