perf stat: Fix crash with --per-node --metric-only in CSV mode

The following command will get segfault due to missing aggr_header_csv
for AGGR_NODE:

  $ sudo perf stat -a --per-node -x, --metric-only true

Committer testing:

Before this patch:

  # perf stat -a --per-node -x, --metric-only true
  Segmentation fault (core dumped)
  #

After:

  # gdb perf
  -bash: gdb: command not found
  # perf stat -a --per-node -x, --metric-only true
  node,Ghz,frontend cycles idle,backend cycles idle,insn per cycle,branch-misses of all branches,
  N0,32,0.335,2.10,0.65,0.69,0.03,1.92,
  #

Fixes: 86895b48

 ("perf stat: Add --per-node agregation support")
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: James Clark <james.clark@arm.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Xing Zhengjun <zhengjun.xing@linux.intel.com>
Link: http://lore.kernel.org/lkml/20221107213314.3239159-2-namhyung@kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>