perf script: Fix crash when processing recorded stat data

While updating perf to work with Python3 and Python2 I noticed that the
stat-cpi script was dumping core.

$ perf  stat -e cycles,instructions record -o /tmp/perf.data /bin/false

 Performance counter stats for '/bin/false':

           802,148      cycles

           604,622      instructions                                                       802,148      cycles
           604,622      instructions

       0.001445842 seconds time elapsed

$ perf script -i /tmp/perf.data -s scripts/python/stat-cpi.py
Segmentation fault (core dumped)
...
...
    rblist=rblist@entry=0xb2a200 <rt_stat>,
    new_entry=new_entry@entry=0x7ffcb755c310) at util/rblist.c:33
    ctx=<optimized out>, type=<optimized out>, create=<optimized out>,
    cpu=<optimized out>, evsel=<optimized out>) at util/stat-shadow.c:118
    ctx=<optimized out>, type=<optimized out>, st=<optimized out>)
    at util/stat-shadow.c:196
    count=count@entry=727442, cpu=cpu@entry=0, st=0xb2a200 <rt_stat>)
    at util/stat-shadow.c:239
    config=config@entry=0xafeb40 <stat_config>,
    counter=counter@entry=0x133c6e0) at util/stat.c:372
...
...

The issue is that since 1fcd0394 perf_stat__update_shadow_stats now calls
update_runtime_stat passing rt_stat rather than calling update_stats but
perf_stat__init_shadow_stats has never been called to initialize rt_stat in
the script path processing recorded stat data.

Since I can't see any reason why perf_stat__init_shadow_stats() is presently
initialized like it is in builtin-script.c::perf_sample__fprint_metric()
[4bd1bef8] I'm proposing it instead be initialized once in __cmd_script

Committer testing:

After applying the patch:

  # perf script -i /tmp/perf.data -s tools/perf/scripts/python/stat-cpi.py
       0.001970: cpu -1, thread -1 -> cpi 1.709079 (1075684/629394)
  #

No segfault.
Signed-off-by: Tony Jones <tonyj@suse.de>
Reviewed-by: Jiri Olsa <jolsa@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Tested-by: Ravi Bangoria <ravi.bangoria@linux.ibm.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jin Yao <yao.jin@linux.intel.com>
Fixes: 1fcd0394 ("perf stat: Update per-thread shadow stats")
Link: http://lkml.kernel.org/r/20190120191414.12925-1-tonyj@suse.deSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/builtin-script.c

@@ -1681,13 +1681,8 @@ static void perf_sample__fprint_metric(struct perf_script *script,
 		.force_header = false,
 	};
 	struct perf_evsel *ev2;
-	static bool init;
 	u64 val;
 
-	if (!init) {
-		perf_stat__init_shadow_stats();
-		init = true;
-	}
 	if (!evsel->stats)
 		perf_evlist__alloc_stats(script->session->evlist, false);
 	if (evsel_script(evsel->leader)->gnum++ == 0)
@@ -2359,6 +2354,8 @@ static int __cmd_script(struct perf_script *script)
 
 	signal(SIGINT, sig_handler);
 
+	perf_stat__init_shadow_stats();
+
 	/* override event processing functions */
 	if (script->show_task_events) {
 		script->tool.comm = process_comm_event;