#6 Check return codes everywhere

[Markus Elfring]

A few checks for return codes are missing. Would you like to add more error handling for return values from "fprintf" and "fclose" in the function "giv_print_svg"?
https://github.com/dov/giv/blob/ff10d17d42aabfd2edbd23c078363245af86a873/src/svg.c#L22