ubik/udebug.c: check array index before dereferencing

Avoid out-of-bounds array accesses by first checking that the index
is in-bounds, and then dereferencing; not the other way around.

Reviewed-on: http://gerrit.openafs.org/8879
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 0e83cde60e6e91fbe20ffac0f0a77afe8497d950)

Change-Id: I7a5018011939d43b702c63a272d53a2018df7d33
Reviewed-on: http://gerrit.openafs.org/9513
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>
Tested-by: BuildBot <buildbot@rampaginggeek.com>