qrexec: handle buffered writes correctly

In case when we have a buffered write, always append to the
buffer, even if the pipe happens to be writable now. If not,
in case of certain tight race we might end up writing buffered data in
wrong order.

qrexec/write_stdin.c

@@ -60,6 +60,12 @@ int write_stdin(int fd, int clid, char *data, int len,
 		struct buffer *buffer)
 {
 	int ret;
+
+	if (buffer_len(buffer)) {
+		buffer_append(buffer, data, len);
+		return WRITE_STDIN_BUFFERED;
+	}
+	
 	ret = write(fd, data, len);
 	if (ret == len)
 		return WRITE_STDIN_OK;