Advance pos_of_buffer_start when writing directly to a writer

If a message is bigger than the buffer, it will be written
directly to the writer, and there was a bug in that case where
pos_of_buffer_start was not advanced. This causes later
assertions to fail, such as the one in
Message::write_length_delimited_to, which asserts based on
total_bytes_written(), which is in turn based on
pos_of_buffer_start.

protobuf/src/coded_output_stream/mod.rs

@@ -244,6 +244,7 @@ impl<'a> CodedOutputStream<'a> {
             }
             OutputTarget::Write(ref mut write, _) => {
                 write.write_all(bytes)?;
+                self.pos_of_buffer_start += bytes.len() as u64;
             }
             OutputTarget::Vec(ref mut vec) => {
                 assert!(self.buffer.pos_within_buf() == 0);