archive-tar: convert snprintf to xsnprintf

archive-tar: convert snprintf to xsnprintf

Commit f2f0267 (archive-tar: use xsnprintf for trivial
formatting, 2015-09-24) converted cases of "sprintf" to
"xsnprintf", but accidentally left one as just "snprintf".
This meant that we could silently truncate the resulting
buffer instead of flagging an error.

In practice, this is impossible to achieve, as we are
formatting a ustar checksum, which can be at most 7
characters. But the point of xsnprintf is to document and
check for "should be impossible" conditions; this site was
just accidentally mis-converted during f2f0267.

Noticed-by: Paul Green <Paul.Green@stratus.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>