tools/testing/selftests/cgroup/cgroup_util.c: cg_read_strcmp: fix null pointer derefe...

tools/testing/selftests/cgroup/cgroup_util.c: cg_read_strcmp: fix null pointer dereference

Haven't reproduced this issue. This PR is does a minor code cleanup.

Signed-off-by: Gaurav Singh <gaurav1086@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: Michal Koutn <mkoutny@suse.com>
Cc: Roman Gushchin <guro@fb.com>
Cc: Christian Brauner <christian.brauner@ubuntu.com>
Cc: Chris Down <chris@chrisdown.name>
Link: http://lkml.kernel.org/r/20200726013808.22242-1-gaurav1086@gmail.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>