cmd/pprof: handle empty profile gracefully

The command "go tool pprof -top $GOROOT/bin/go /dev/null" now logs that
profile is empty instead of panicking.

Fixes #9207

Change-Id: I3d55c179277cb19ad52c8f24f1aca85db53ee08d
Reviewed-on: https://go-review.googlesource.com/2571
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/pprof/internal/driver/driver.go

@@ -1013,6 +1013,10 @@ func generate(interactive bool, prof *profile.Profile, obj plugin.ObjTool, ui pl
 		w = outputFile
 	}
 
+	if prof.Empty() {
+		return fmt.Errorf("profile is empty")
+	}
+
 	value, stype, unit := sampleFormat(prof, f)
 	o.SampleType = stype
 	rpt := report.New(prof, *o, value, unit)

src/cmd/pprof/internal/profile/profile.go

@@ -565,3 +565,8 @@ func (p *Profile) Demangle(d Demangler) error {
 	}
 	return nil
 }
+
+// Empty returns true if the profile contains no samples.
+func (p *Profile) Empty() bool {
+	return len(p.Sample) == 0
+}

src/cmd/pprof/internal/profile/profile_test.go

@@ -0,0 +1,24 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package profile
+
+import (
+	"bytes"
+	"testing"
+)
+
+func TestEmptyProfile(t *testing.T) {
+	var buf bytes.Buffer
+	p, err := Parse(&buf)
+	if err != nil {
+		t.Error("Want no error, got", err)
+	}
+	if p == nil {
+		t.Fatal("Want a valid profile, got <nil>")
+	}
+	if !p.Empty() {
+		t.Errorf("Profile should be empty, got %#v", p)
+	}
+}