multiboot: bss_end_addr can be zero

multiboot: bss_end_addr can be zero

The multiboot spec (https://www.gnu.org/software/grub/manual/multiboot/),
section 3.1.3, allows for bss_end_addr to be zero.

A zero bss_end_addr signifies there is no .bss section.

Suggested-by: Daniel Kiper <daniel.kiper@oracle.com>
Signed-off-by: Jack Schwartz <jack.schwartz@oracle.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
Reviewed-by: Prasad J Pandit <pjp@fedoraproject.org>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>