relocator: Protect grub_relocator_alloc_chunk_addr() input args against integer underflow/overflow
    
    Use arithmetic macros from safemath.h to accomplish it. In this commit,
    I didn't want to be too paranoid to check every possible math equation
    for overflow/underflow. Only obvious places (with non zero chance of
    overflow/underflow) were refactored.
    
    Signed-off-by: Alexey Makhalov <amakhalov@vmware.com>
    Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>