[hdprefix] Avoid attempts to read beyond the end of the disk

[hdprefix] Avoid attempts to read beyond the end of the disk

When booting from a hard disk image (e.g. bin/ipxe.usb) within an
emulator such as QEMU, the disk may not exist beyond the end of the
image.  Limit all reads to the length of the image to avoid spurious
errors when loading the iPXE image.

Signed-off-by: Michael Brown <mcb30@ipxe.org>