perf evsel: Make perf_evsel__name() accept a NULL argument

In which case it simply returns "unknown", like when it can't figure out
the evsel->name value.

This makes this code more robust and fixes a problem in 'perf trace'
where a NULL evsel was being passed to a routine that only used the
evsel for printing its name when a invalid syscall id was passed.

Reported-by: Leo Yan <[e-mail]>
Cc: Adrian Hunter <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Link: https://lkml.kernel.org/n/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/util/evsel.c

···
 {
 	char bf[128];
 
+	if (!evsel)
+		goto out_unknown;
+
 	if (evsel->name)
 		return evsel->name;
 
···
 
 	evsel->name = strdup(bf);
 
-	return evsel->name ?: "unknown";
+	if (evsel->name)
+		return evsel->name;
+out_unknown:
+	return "unknown";
 }
 
 const char *perf_evsel__group_name(struct perf_evsel *evsel)