Tar bidder should just return a zero bid ("not me!") if
it sees a truncated input the first time it gets called.
(In particular, files shorter than 512 bytes cannot be tar archives.)
This allows the top-level archive_read_next_header code to
generate a proper error message for unrecognized file types.

Pointed out by: numerous ports that expect tar to extract non-tar files ;-(
Thanks to: Kris Kennaway