auth: Fix buffer overflow in afsconf_Open

If we fallback to the .AFSCONF file in the user's homedirectory,
the results of getenv("HOME") are copied into a fixed length string,
without checking for overflows.

Instead of risking this, just use asprintf to dynamically construct
a string, and free it when we are done.

Caught by coverity (#985905)

Change-Id: Id8769ede841165d3ff3104143e55767d550d6f87
Reviewed-on: http://gerrit.openafs.org/9292
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>