Fix invalid channel ID handling.

Fix the issue where get_state* functions do not handle when id is RELAY_CHN_ID_ALL.
Fixes #1037.

include/relay_chn.h

@@ -49,6 +49,7 @@ typedef enum relay_chn_direction_enum relay_chn_direction_t;
  * @brief Enums that represent the state of a relay channel.
  */
 enum relay_chn_state_enum {
+    RELAY_CHN_STATE_UNDEFINED,  ///< The relay channel state is undefined.
     RELAY_CHN_STATE_FREE,       ///< The relay channel is free to run or execute commands.
     RELAY_CHN_STATE_STOPPED,    ///< The relay channel is stopped and not running.
     RELAY_CHN_STATE_FORWARD,    ///< The relay channel is running in the forward direction.