gkm: Fix some format-truncation warnings

When enabling fatal warnings (like in our CI), the -Werror=format-truncation= flag is complaining about us using snprintf() in a way that can theoretically truncate its arguments, even though this will not happen in practice, unless people are still running gnome-keyring in the year 100000.

Solve the warning by doing a dumb check that aborts if snprintf returns an error