perf test: Fix hists related entries

That got broken by d3a72fd8 ("perf report: Fix indentation of
dynamic entries in hierarchy"), by using the evlist in setup_sorting()
without checking if it is NULL, as done in some 'perf test' entries:

  $ find tools/ -name "*.c" | xargs grep 'setup_sorting(NULL);'
  tools/perf/tests/hists_output.c:      setup_sorting(NULL);
  tools/perf/tests/hists_output.c:      setup_sorting(NULL);
  tools/perf/tests/hists_output.c:      setup_sorting(NULL);
  tools/perf/tests/hists_output.c:      setup_sorting(NULL);
  tools/perf/tests/hists_output.c:      setup_sorting(NULL);
  tools/perf/tests/hists_cumulate.c:    setup_sorting(NULL);
  tools/perf/tests/hists_cumulate.c:    setup_sorting(NULL);
  tools/perf/tests/hists_cumulate.c:    setup_sorting(NULL);
  tools/perf/tests/hists_cumulate.c:    setup_sorting(NULL);
  $

Fix it.

Before:

  [root@jouet ~]# perf test
  <SNIP>
  15: Test matching and linking multiple hists                 : FAILED!
  16: Try 'import perf' in python, checking link problems      : Ok
  17: Test breakpoint overflow signal handler                  : Ok
  18: Test breakpoint overflow sampling                        : Ok
  19: Test number of exit event of a simple workload           : Ok
  20: Test software clock events have valid period values      : Ok
  21: Test object code reading                                 : Ok
  22: Test sample parsing                                      : Ok
  23: Test using a dummy software event to keep tracking       : Ok
  24: Test parsing with no sample_id_all bit set               : Ok
  25: Test filtering hist entries                              : FAILED!
  26: Test mmap thread lookup                                  : Ok
  27: Test thread mg sharing                                   : Ok
  28: Test output sorting of hist entries                      : FAILED!
  29: Test cumulation of child hist entries                    : FAILED!
  <SNIP>

After the patch the above failed tests complete successfully.

Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Fixes: d3a72fd8 ("perf report: Fix indentation of dynamic entries in hierarchy")
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>