MT#59962 ModConference: do not leave allocated chan

If a channel has been allocated with
`ConferenceJoinChannel()` then remember to transfer
the ownership over it to the DSM sessions's
"garbage collector" which later takes care to
deallocate it.

Fixes:

    CID 542153: (#1 of 1): Resource leak (RESOURCE_LEAK)
    10. leaked_storage: Variable chan going out of scope leaks the storage it points to.

Change-Id: Icdc2c67cf2a349ec70fd83f1cf6a43ecfcf69547

apps/dsm/mods/mod_conference/ModConference.cpp

@@ -229,7 +229,15 @@ EXEC_ACTION_START(ConfRejoinAction) {
   }
 
   if (ConferenceJoinChannel(&chan, sess, sc_sess, channel_id, mode)) {
-  sc_sess->CLR_ERRNO;
+    /* save channel for later use */
+    AmArg c_arg;
+    c_arg.setBorrowedPointer(chan);
+    sc_sess->avar[CONF_AKEY_CHANNEL] = std::move(c_arg);
+
+    /* add to garbage collector */
+    sc_sess->transferOwnership(chan);
+
+    sc_sess->CLR_ERRNO;
   } else {
     sc_sess->SET_ERRNO(DSM_ERRNO_UNKNOWN_ARG);
   }