perf parse-events: Use zfree() to reduce chances of use after free

Do defensive programming by using zfree() to initialize freed pointers
to NULL, so that eventual use after free result in a NULL pointer deref
instead of more subtle behaviour.

Also remove one NULL test before free(), as it accepts a NULL arg and we
get one line shaved not doing it explicitely.
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/parse-events.c

@@ -2020,7 +2020,7 @@ int perf_pmu__test_parse_init(void)
 
 err_free:
 	for (j = 0, tmp = list; j < i; j++, tmp++)
-		free(tmp->symbol);
+		zfree(&tmp->symbol);
 	free(list);
 	return -ENOMEM;
 }