Don't use 0 for NULL in C

It may be desirable in C++, but not in C

Change-Id: I2d1c376f62563b4ed038e447f591e5afa10e15b8
Reviewed-on: http://review.couchbase.org/34929
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>