aklog: Fix segfault on aklog -path

Commit 2fac53522e7ef5b3a376e191bffdc1f6784e6995 “aklog: Fix improper
use of readlink” inadvertently changed the meaning of int link from a
boolean flag (length > 0) to just a length.  This caused ‘aklog -path
(anything)’ to segfault.

Update the type of link and the condition of the while loop to account
for this change.

FIXES 131930

Change-Id: Ia05836795425a53e858ab29866900f6d45970644
Reviewed-on: http://gerrit.openafs.org/11517
Reviewed-by: Anders Kaseorg <andersk@mit.edu>
Tested-by: Anders Kaseorg <andersk@mit.edu>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>
Tested-by: Stephan Wiesand <stephan.wiesand@desy.de>
Reviewed-by: D Brashear <shadow@your-file-system.com>