prdb_check: fix out of bounds array access in continuation entries

prdb_check: fix out of bounds array access in continuation entries

A continuation entry (struct contentry) contains 39 id elements, however
a regular entry (struct prentry) contains only 10 id elements.
Attempting to access more than 10 elements of a regular entry is
undefined behavior.

Use a stuct contentry when when processing continuation entries in
prdb_check.  This is done to safely traverse the id arrays of the
continuation entries.  Use the new pr_PrintContEntry to print
continuation entries.

The undefined behavior manfests as a segmentation violation in
WalkNextChain() when built with GCC 4.8 with optimization enabled.

Change-Id: I7613345ee6b7b232c5a0645f4f302c3eac0cdc15
Reviewed-on: http://gerrit.openafs.org/11742
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>