go/mcap: Fix scenario where schemaID equals 0

go/mcap/reader_test.go

@@ -21,7 +21,7 @@ func TestIndexedReaderBreaksTiesOnChunkOffset(t *testing.T) {
 	assert.Nil(t, err)
 	assert.Nil(t, writer.WriteHeader(&Header{}))
 	assert.Nil(t, writer.WriteSchema(&Schema{
-		ID:       0,
+		ID:       1,
 		Name:     "",
 		Encoding: "",
 		Data:     []byte{},
@@ -586,7 +586,7 @@ func TestReadingMessageOrderWithOverlappingChunks(t *testing.T) {
 	assert.Nil(t, err)
 	assert.Nil(t, writer.WriteHeader(&Header{}))
 	assert.Nil(t, writer.WriteSchema(&Schema{
-		ID:       0,
+		ID:       1,
 		Name:     "",
 		Encoding: "",
 		Data:     []byte{},

go/mcap/writer.go

@@ -104,6 +104,12 @@ func (w *Writer) WriteFooter(f *Footer) error {
 // identified within a file by their schema ID. A Schema record must occur at
 // least once in the file prior to any Channel Info referring to its ID.
 func (w *Writer) WriteSchema(s *Schema) (err error) {
+	if s == nil {
+		return errors.New("schema struct can not be nil")
+	}
+	if s.ID == 0 {
+		return errors.New("schemaID must not be zero")
+	}
 	msglen := 2 + 4 + len(s.Name) + 4 + len(s.Encoding) + 4 + len(s.Data)
 	w.ensureSized(msglen)
 	offset := putUint16(w.msg, s.ID)