[PATCH] erofs-utils: tar: fix negative GNU base-256 number parsing

[Vansh Choudhary]

Yes, this definitely fixes the signed-overflow UB in tarerofs_parsenum(),
and also makes the negative GNU base-256 decoding logic correct.  But
from the current testcase alone, I cannot firmly show a visible end-user
failure on the old code path.

So I think this is safest to describe as a correctness + UB fix.

Thanks,
Vansh