perf report: Tell the user when a perf.data file has no samples

[root@emilia ~]# perf report --stdio
The perf.data file has no samples!
[root@emilia ~]#

The TUI shows a popup warning message with the same message.

Reported-by: Ingo Molnar <mingo@elte.hu>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Tom Zanussi <tzanussi@gmail.com>
LKML-Reference: <new-submission>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/builtin-report.c

@@ -350,6 +350,12 @@ static int __cmd_report(void)
 		perf_session__fprintf_dsos(session, stdout);
 
 	next = rb_first(&session->hists_tree);
+
+	if (next == NULL) {
+		ui__warning("The %s file has no samples!\n", input_name);
+		goto out_delete;
+	}
+
 	while (next) {
 		struct hists *hists;