perf evlist: Destructors should accept NULL

And do nothing, just like free(), to avoid having to test it in callers,
usually in error paths.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-mexbavy0ft387j5w89t365eu@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/tests/parse-no-sample-id-all.c

@@ -44,8 +44,7 @@ static int process_events(union perf_event **events, size_t count)
 	for (i = 0; i < count && !err; i++)
 		err = process_event(&evlist, events[i]);
 
-	if (evlist)
-		perf_evlist__delete(evlist);
+	perf_evlist__delete(evlist);
 
 	return err;
 }