libefivar: simplify efi_name_to_guid()'s string termination.
libefivar: simplify efi_name_to_guid()'s string termination.

There's no reason to try to set the whole string end to 0 when just the
character in question will do, and the arithmetic is way easier this
way.

Signed-off-by: Peter Jones <2e0a021e603479ff81838c00c3b770daca4e0214@redhat.com>

vathpela/efivar-devel,rhinstaller/efivar,rhboot/efivar,rhinstaller/efivar,rhboot/efivar