afsd: Avoid dir interpolation for memcache

afsd: Avoid dir interpolation for memcache

memcache doesn't make use of fullpn_DCacheFile, fullpn_VolInfoFile,
etc. Do not even try to generate these strings for memcache, since
cacheBaseDir will be NULL, and so this can cause a segfault on some
platforms including Solaris.

Reviewed-on: http://gerrit.openafs.org/7442
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit 2e2bc535bd03df86f78b648f78d2d0a95d273fab)

Change-Id: I46f2da9b2f0e5f8d004ebd3d799a3c1ac22c3e95
Reviewed-on: http://gerrit.openafs.org/7501
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>