grub2: fix empty file handling

Currently, we have a bug when parsing zero-length files: we subtract one
from the length to exclude the trailing NUL (added by read_file), but a
zero-length file will result in a length of -1.

This change adds an explicit exit if we're attempting to parse an empty
file.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>