Fix FILE resource leak

[nmcclatchey]

Fixes resource leaks in npy_load() and npz_load() that would occur if an exception were to be thrown during load. Because NPY files can be constructed deliberately to trigger such exceptions, addressing this leak is vital for cases where programs may load user-supplied NPY files and attempt recovery on error.

[nmcclatchey]

Note: Resolves #52 and #53 .

[s-trinh]

Thanks for the code. I have used it with the following changes:

  FILE *fp;
  AutoCloser() : fp(nullptr) { }
  ~AutoCloser()
  {
    if (fp != nullptr) {
      fclose(fp);
    }
  }
};

I have added the constructor, otherwise my compiler complains about fp not being initialized.
The fp != nullptr is probably useless but it looks like sending NULL (so also nullptr?) to fclose is undefined behavior.