internal/format: don't generate and reject empty lines in recipient bodies

Detected by https://github.com/str4d/rage/runs/532262359 and by go-fuzz.

cmd/age/age_test.go

@@ -0,0 +1,47 @@
+// Copyright 2019 Google LLC
+//
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file or at
+// https://developers.google.com/open-source/licenses/bsd
+
+package main
+
+import (
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"filippo.io/age/internal/age"
+)
+
+func TestVectors(t *testing.T) {
+	files, _ := filepath.Glob("testdata/*.age")
+	for _, f := range files {
+		name := strings.TrimSuffix(strings.TrimPrefix(f, "testdata/"), ".age")
+		t.Run(name, func(t *testing.T) {
+			identities, err := parseIdentitiesFile("testdata/" + name + "_key.txt")
+			if err != nil {
+				t.Fatal(err)
+			}
+			for _, i := range identities {
+				t.Logf("%s", i.Type())
+			}
+
+			in, err := os.Open("testdata/" + name + ".age")
+			if err != nil {
+				t.Fatal(err)
+			}
+			r, err := age.Decrypt(in, identities...)
+			if err != nil {
+				t.Fatal(err)
+			}
+			out, err := ioutil.ReadAll(r)
+			if err != nil {
+				t.Fatal(err)
+			}
+			t.Logf("%s", out)
+		})
+	}
+}

cmd/age/testdata/empty_recipient_body.age

@@ -0,0 +1,6 @@
+age-encryption.org/v1
+-> ssh-ed25519 o1Hudg SZISkI5Qn8YgUBmTKG/Zp/QpFjXWvAivzvB+hOcN5W8
+dYfwGWYvCwpSU5EXIC1XqfXdsBvCi3kMypdqCVShrpk
+-> joint-oil-hw
+--- gC/27VAgqOEzAQMKHvBjih7sJ1oDKht+HNdguTIbjt8
+fëtAeµÖ¨&8{Ëéðνcat—íΘœ¯šË·}«=šC†Ÿu

cmd/age/testdata/empty_recipient_body_key.txt

@@ -0,0 +1,7 @@
+-----BEGIN OPENSSH PRIVATE KEY-----
+b3BlbnNzaC1rZXktdjEAAAAABG5vbmUAAAAEbm9uZQAAAAAAAAABAAAAMwAAAAtzc2gtZW
+QyNTUxOQAAACAwKgrb/LkvtI887QylSoUh5xUlKr1fb37euR6et5jHowAAAJgxqUx+MalM
+fgAAAAtzc2gtZWQyNTUxOQAAACAwKgrb/LkvtI887QylSoUh5xUlKr1fb37euR6et5jHow
+AAAEC7gKj74YIwaM1BT2tnODjfeZJvo8lcazvL6Uljv3+nIDAqCtv8uS+0jzztDKVKhSHn
+FSUqvV9vft65Hp63mMejAAAADnJ1bm5lckBmdi1hejMyAQIDBAUGBw==
+-----END OPENSSH PRIVATE KEY-----