Don't allocate objects of 0 length

Fix assorted places in the code where we might have asked malloc to
allocate a block of zero length.

Caught by clang-analyzer

Change-Id: I6e1226ad83a52984ee7c53cbed8c867f38e4f866
Reviewed-on: http://gerrit.openafs.org/9155
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>