iniexample segfault if no write rights

I went into a error segfault : 
```
# cd /
# /home/user/iniparser/test/iniexample
zsh: segmentation fault  /home/user/iniparser/test/iniexample
```

The trouble is an user cannot create a file in the /, but the call to fopen is not checked !
Then a NULL pointer is used further in the function and lead to a segfault

I set a small fix : 
```
# cd /
# /home/user/iniparser/test/iniexample
Cannot create example.ini (Permission denied)
iniparser: cannot open example.ini
cannot parse file: example.ini
```