+/** @brief Convert a hex string to bytes
+ * @param s Pointer to hex string
+ * @param np Where to store byte string length or NULL
+ * @return Allocated buffer, or 0
+ *
+ * @p s should point to a 0-terminated string containing an even number
+ * of hex digits. They are converted to bytes and returned via the return
+ * value and optionally the length via @p np.
+ *
+ * On any error a message is logged and a null pointer is returned.
+ */