The "perf test Session topology" entry fails with core dump on s390. The root
cause is a NULL pointer dereference in function check_cpu_topology() line 76
(or line 82 without -v).

The session->header.env.cpu variable is NULL because on s390 function
process_cpu_topology() returns with error:

    socket_id number is too big.
    You may need to upgrade the perf tool.

and releases the env.cpu variable via zfree() and sets it to NULL.

Here is the gdb output:
(gdb) n
76                      pr_debug("CPU %d, core %d, socket %d\n", i,
(gdb) n

Program received signal SIGSEGV, Segmentation fault.
0x00000000010f4d9e in check_cpu_topology (path=0x3ffffffd6c8
	"/tmp/perf-test-J6CHMa", map=0x14a1740) at tests/topology.c:76
76  pr_debug("CPU %d, core %d, socket %d\n", i,
(gdb)

Make sure the env.cpu variable is not used when its NULL.
Test for NULL pointer and return TEST_SKIP if so.

Output before:

  [root@p23lp27 perf]# ./perf test -F 39
  39: Session topology  :Segmentation fault (core dumped)
  [root@p23lp27 perf]#

Output after:

  [root@p23lp27 perf]# ./perf test -vF 39
  39: Session topology                                      :
  --- start ---
  templ file: /tmp/perf-test-Ajx59D
  socket_id number is too big.You may need to upgrade the perf tool.
  ---- end ----
  Session topology: Skip
  [root@p23lp27 perf]#
Signed-off-by: Thomas Richter <tmricht@linux.ibm.com>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Link: http://lkml.kernel.org/r/20180528073657.11743-1-tmricht@linux.ibm.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>