go.text/language: fixed bug that caused EncodeM49 to panic for some values.

The added test doesn't test much, but mainly serves the purpose of asserting
no panic occurs for any value.

R=r
CC=golang-codereviews
https://golang.org/cl/48080043

language/language_test.go

@@ -207,6 +207,11 @@ func TestEncodeM49(t *testing.T) {
 			t.Errorf("%d:%d: was %s, %v; want %s, %v", i, tt.m49, r, err == nil, tt.code, tt.ok)
 		}
 	}
+	for i := 1; i <= 1000; i++ {
+		if r, err := EncodeM49(i); err == nil && r.M49() == 0 {
+			t.Errorf("%d has no error, but maps to undefined region", i)
+		}
+	}
 }
 
 func TestParseRegion(t *testing.T) {

language/lookup.go

@@ -296,7 +296,7 @@ func getRegionM49(n int) (regionID, error) {
 		i := sort.Search(len(buf), func(i int) bool {
 			return buf[i] >= val
 		})
-		if r := buf[i]; r&^regionMask == val {
+		if r := fromM49[int(m49Index[idx])+i]; r&^regionMask == val {
 			return regionID(r & regionMask), nil
 		}
 	}