dumpscan: appease gcc8 -Wformat-overflow

dumpscan: appease gcc8 -Wformat-overflow

gcc does not benefit from our external knowledge that tm_year is
tightly bounded, and thinks it could still be in the range
[-2147481748, 2147483647], which would overflow our string buffer.
The function in question does not have error handling in place, so
rather than adding some or trying to assert the proper bounds, just
use a slightly larger buffer for safety.

Reviewed-on: https://gerrit.openafs.org/13472
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Cheyenne Wills <cwills@sinenomine.net>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit 8f03ff3bdd8eb9f4557cdb7054aee9b8ea432160)

Change-Id: I05c8d998c6d40118a1bde923e346cddbdfa4192b
Reviewed-on: https://gerrit.openafs.org/13741
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Marcio Brito Barbosa <mbarbosa@sinenomine.net>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>