multiboot: Check validity of mh_header_addr
    
    I couldn't find a case where this prevents something bad from happening
    that isn't already caught by other checks, but let's err on the safe
    side and check that mh_header_addr is as expected.
    
    Signed-off-by: Kevin Wolf <kwolf@redhat.com>
    Reviewed-by: Jack Schwartz <jack.schwartz@oracle.com>
    (cherry picked from commit dbf2dce7aabb7723542bd182175904846d70b0f9)
    Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>