bos: Don't overflow buffer with key data

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)

Change-Id: I44fb62d30c5022e650475b3ca51a28bcb7cf1e06
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>