cgtop: initialize `ours' to NULL properly (#3139)

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.

src/cgtop/cgtop.c

@@ -362,7 +362,7 @@ static int refresh_one(
                 Group **ret) {
 
         _cleanup_closedir_ DIR *d = NULL;
-        Group *ours;
+        Group *ours = NULL;
         int r;
 
         assert(controller);