Running cgtop on a system, which lacks expecting stat file, results in a
segfault. For example, a system with blkio tree but without cfq io scheduler,
lacks "blkio.io_service_bytes".
When the targeting cgroup's file does not exist, process() returns 0 and
also does not modify `*ret' value (which is `*ours'). As a result,
callers of refresh_one() can have bogus pointer, which result in SEGV.
This patch just properly initialize the variable to NULL.
80b2ab4bc0