Reset the use_count to zero in chan_free

It is possible that the BTS is closing the channel even when
our upper layers are doing work. Reset the use_count add a fixme
to call cancellations for pending operations. Cancellation of the
call state (state machines in general) and such come into mind...

src/chan_alloc.c

@@ -193,6 +193,11 @@ void lchan_free(struct gsm_lchan *lchan)
 		lchan->subscr = 0;
 	}
 
+	/* We might kill an active channel... FIXME: call cancellations */
+	if (lchan->use_count != 0) {
+		lchan->use_count = 0;
+	}
+
 	/* stop the timer */
 	del_timer(&lchan->release_timer);