Always clear the destination string in copyinstr() on error, unless we faulted when writing to the first character.
Always clear the destination string in copyinstr() on error, unless we faulted
when writing to the first character.

With some help from millert@.

orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars