bos: Don't overflow buffer with key data

When parsing key data from the command line, don't overflow the
buffer used to hold it - instead just give an error if the data
is too long.

Caught by coverity (#985775)

Reviewed-on: http://gerrit.openafs.org/9550
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit 4e9c6eb9d5192888d79a07042c9cb6029def9726)

Change-Id: Ic1892ba4cd5e69c48003073a758a47a08b84a890
Reviewed-on: http://gerrit.openafs.org/10861
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Perry Ruiter <pruiter@sinenomine.net>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>