main/busybox: du size overflow

busybox du stores the size sum as unsigned long long. This works until ~16 Exabytes (2^64-1) but overflows beyond that due to missing overflow check. That may be unlikely but sparse files can do that.

(Run in a tmpfs. Otherwise dd may throw size errors)

dd if=/dev/null bs=1 seek=$(echo '2^63-1' | bc) of=a
dd if=/dev/null bs=1 seek=$(echo '2^63-1' | bc) of=b
printf 12 > more
busybox du -bc a b
=> 18446744073709551614	total
busybox du -bc a b more
=> 0	total

The du implementation of coreutils returns Infinity (instead of 0) and a 0 exit code. I don't like the approach of coreutils. I think du should return a non zero exit code if it can't return the correct size and then the printed value is irrelevant.

It seems unlikely that this can cause real issues but may be worth fixing.

CC @nmeum