unlog: Don't overflow cells array

cells has a maximum size of MAXCELLS. Doing cells[MAXCELLS] overflows
that array. Clamp our maximum number of cells at one below this to
avoid the overflow.

Caught by coverity (#98551)

Reviewed-on: http://gerrit.openafs.org/9324
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
(cherry picked from commit ff7fd58b49a41bf85df492b9f470004d4433800e)

Change-Id: If6c0caed4b4fedf571c4c6c6b8466bfaaff67975
Reviewed-on: http://gerrit.openafs.org/9374
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Paul Smeddle <paul.smeddle@gmail.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>