Check for integer overflow in osGetMBlocks

Fixes Trac #5188.

Signed-off-by: Austin Seipp <aseipp@pobox.com>
Paranoid integer overflow check in my_mmap

Signed-off-by: Austin Seipp <aseipp@pobox.com>
Paranoid integer overflow check in osGetMBlocks

Signed-off-by: Austin Seipp <aseipp@pobox.com>