subscriptions: stop re-entrant call to subs_reschedule()

THe problem is that mux subs have been done such that unsubscribe
(which in turn calls subs_reschedule()) can itelf be called in line
from subs_reschedule().

Really this is a bit of a mess but this simple hack fixes things.

src/subscriptions.c

@@ -185,6 +185,7 @@ subscription_reschedule(void)
   streaming_message_t *sm;
   int error;
   if (reenter) return;
+  reenter = 1;
 
   lock_assert(&global_lock);