perf parse-events: Fix segfault when event parser gets an error

parse_events() is often called with parse_events_error set to NULL.
Make parse_events_error__handle() not segfault in that case.

A subsequent patch changes to avoid passing NULL in the first place.

Fixes: 43eb05d066795bdf ("perf tests: Support 'Track with sched_switch' test for hybrid")
Signed-off-by: Adrian Hunter <[e-mail]>
Cc: Ian Rogers <[e-mail]>
Cc: Jin Yao <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/util/parse-events.c

···
 void parse_events_error__handle(struct parse_events_error *err, int idx,
 				char *str, char *help)
 {
-	if (WARN(!str, "WARNING: failed to provide error string\n")) {
-		free(help);
-		return;
+	if (WARN(!str, "WARNING: failed to provide error string\n"))
+		goto out_free;
+	if (!err) {
+		/* Assume caller does not want message printed */
+		pr_debug("event syntax error: %s\n", str);
+		goto out_free;
 	}
 	switch (err->num_errors) {
 	case 0:
···
 		break;
 	}
 	err->num_errors++;
+	return;
+
+out_free:
+	free(str);
+	free(help);
 }
 
 #define MAX_WIDTH 1000