compilation error due to bug in savevm.c

Since 

302dfbeb21fc5154c24ca50d296e865a3778c7da

Add xbzrle_encode_buffer and xbzrle_decode_buffer functions
    
    For performance we are encoding long word at a time.
    For nzrun we use long-word-at-a-time NULL-detection tricks from strcmp():
    using ((lword - 0x0101010101010101) & (~lword) & 0x8080808080808080) test
    to find out if any byte in the long word is zero.
    
    Signed-off-by: Benoit Hudzia <email address hidden>
    Signed-off-by: Petter Svard <email address hidden>
    Signed-off-by: Aidan Shribman <email address hidden>
    Signed-off-by: Orit Wasserman <email address hidden>
    Signed-off-by: Eric Blake <email address hidden>
    
    Reviewed-by: Luiz Capitulino <email address hidden>
    Reviewed-by: Eric Blake <email address hidden>

 commit arrived into master barnch, I can't compile qemu at all:

savevm.c:2476:13: error: overflow in implicit constant conversion [-Werror=overflow]