bridge_basic: don't cache xferfailsound during an attended transfer

The xferfailsound was read from the channel at the beginning of the transfer,
and that value is "cached" for the duration of the transfer. Therefore, changing
the xferfailsound on the channel using the FEATURE() dialplan function does
nothing once the transfer is under way.

This makes it so the transfer code instead gets the xferfailsound configuration
options from the channel when it is actually going to be used.

This patch also fixes a potential memory leak of the props object as well as
making sure the condition variable gets initialized before being destroyed.

ASTERISK-25696 #close

Change-Id: Ic726b0f54ef588bd9c9c67f4b0e4d787934f85e4

include/asterisk/features_config.h

@@ -101,6 +101,21 @@ struct ast_features_xfer_config {
  */
 struct ast_features_xfer_config *ast_get_chan_features_xfer_config(struct ast_channel *chan);
 
+/*!
+ * \brief Get the transfer configuration option xferfailsound
+ *
+ * \note The channel should be locked before calling this function.
+ * \note The returned value has to be freed.
+ *
+ * If no channel is provided, then option is pulled from the global
+ * transfer configuration.
+ *
+ * \param chan The channel to get configuration options for
+ * \retval NULL Failed to get configuration
+ * \retval non-NULL The xferfailsound
+ */
+char *ast_get_chan_features_xferfailsound(struct ast_channel *chan);
+
 /*!
  * \brief Configuration relating to call pickup
  */