message/pipeline: detect unknown keys

When (d *dictionary) Lookup() is called on an non-existing key, the
`messageKeyToIndex[key]` returns 0. As 0 is also a valid entry in
messageKeyToIndex, the method returned the wrong value rather than fail.

Fixes golang/go#35587

Change-Id: Iedd1cf42f29335c2c2052b07993d7f2dfcd3cc6c
Reviewed-on: https://go-review.googlesource.com/c/text/+/207217
Run-TryBot: Jason A. Donenfeld <Jason@zx2c4.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>

message/pipeline/generate.go

@@ -297,7 +297,10 @@ type dictionary struct {
 }
 
 func (d *dictionary) Lookup(key string) (data string, ok bool) {
-	p := messageKeyToIndex[key]
+	p, ok := messageKeyToIndex[key]
+	if !ok {
+		return "", false
+	}
 	start, end := d.index[p], d.index[p+1]
 	if start == end {
 		return "", false

message/pipeline/testdata/test1/catalog_gen.go.want

@@ -14,7 +14,10 @@ type dictionary struct {
 }
 
 func (d *dictionary) Lookup(key string) (data string, ok bool) {
-	p := messageKeyToIndex[key]
+	p, ok := messageKeyToIndex[key]
+	if !ok {
+		return "", false
+	}
 	start, end := d.index[p], d.index[p+1]
 	if start == end {
 		return "", false

message/pipeline/testdata/test1/catalog_test.go

@@ -22,6 +22,10 @@ func TestCatalog(t *testing.T) {
 		lang: "en",
 		key:  "Hello world!\n",
 		want: "Hello world!\n",
+	}, {
+		lang: "en",
+		key:  "non-existing-key\n",
+		want: "non-existing-key\n",
 	}, {
 		lang: "de",
 		key:  "Hello world!\n",