Completion of error handling

Would you like to add more error handling for return values from functions like the following?
- [fseek](http://pubs.opengroup.org/onlinepubs/9699919799/functions/fseek.html "Reposition a file-position indicator in a stream") ⇒ [sha256_file](https://github.com/probonopd/AppImageKit/blob/9329a824e2a2d16762e90d339d696262654f382d/digest.c#L19)
- [malloc](http://pubs.opengroup.org/onlinepubs/9699919799/functions/malloc.html "Memory allocation") ⇒ [main](https://github.com/probonopd/AppImageKit/blob/d655e74204ba857a702a5297d706a9dc1f1b9155/AppRun.c#L50)