ptserver: testpt.c format-overflow warning

ptserver: testpt.c format-overflow warning

GCC 9 introduced new warnings/errors and is flagging a sprintf with a
format-overflow warning.  With --checking-enabled, this error is causing
testpt.c to fail during compile.

Change the buffer size from 16 bytes to PR_MAXNAMELEN+1 and use snprintf
instead of sprintf. Generate an error message and exit if snprintf
truncates the string.

Reviewed-on: https://gerrit.openafs.org/13663
Reviewed-by: Cheyenne Wills <cwills@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit 4a57cc54dfb6789a86ee735360ee44209c1a901a)

Change-Id: I2f8012e7fb4384f3ad877d2c9beb5f00b03716b8
Reviewed-on: https://gerrit.openafs.org/13730
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>