perf script: Fix segfault using --show-mmap-events

Patch "perf script: Don't assume evsel position of tracking events"
changed 'perf script' to use 'perf_evlist__id2evsel()'. That results
in a segfault if there is more than 1 event and there are
synthesized mmap events e.g.

	$ perf record -e cycles,instructions -p$$ sleep 1
	$ perf script --show-mmap-events
	Segmentation fault (core dumped)

That happens because these synthesized events have an 'id' of zero
which does not match any 'evsel'.

Currently, these synthesized events use the sample type of the first
evsel.

Change 'perf_evlist__id2evsel()' to reflect that which also makes
it consistent with 'perf_evlist__event2evsel()'.
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Fixes: 06b234ec ("perf script: Don't assume evsel position of tracking events")
Cc: Jiri Olsa <jolsa@redhat.com>
Link: http://lkml.kernel.org/r/1440059205-1765-1-git-send-email-adrian.hunter@intel.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/evlist.c

@@ -573,7 +573,7 @@ struct perf_evsel *perf_evlist__id2evsel(struct perf_evlist *evlist, u64 id)
 {
 	struct perf_sample_id *sid;
 
-	if (evlist->nr_entries == 1)
+	if (evlist->nr_entries == 1 || !id)
 		return perf_evlist__first(evlist);
 
 	sid = perf_evlist__id2sid(evlist, id);