Fix null-deref crash on subscribe/unsubscribe/event to unknown subscription id

subscriptions[id] via operator[] default-constructed (NULL) and
inserted an entry for any unregistered id, and the very next line
dereferenced it unconditionally - any client referencing a
subscription id that was never created (e.g. Zephora client sending
subscribe:invite for a channel nobody ever Publish()d) segfaulted the
entire process, not just that connection. Use find() and return
gracefully instead.

SubscriptionManager.cpp

@@ -101,7 +101,24 @@ namespace core {
          return 1;
       }
 
-      auto subscription = subscriptions[request[1].str()];
+      // Was subscriptions[request[1].str()] - map::operator[] default-
+      // constructs (here: NULL) and INSERTS an entry for any id that
+      // isn't already registered, and subscription->id below was then
+      // dereferenced completely unconditionally. Any subscribe:/
+      // unsubscribe:/event: naming an id nobody ever created (confirmed
+      // live: "subscribe:invite", sent by the client on every scene
+      // load, referencing an "invite" subscription that plain doesn't
+      // exist anywhere - real invites are Publish()'d under a
+      // per-group-guid id, not a shared "invite" channel) null-derefs
+      // and takes down the ENTIRE process, every connection included,
+      // not just the one that sent it. find() avoids polluting the map
+      // with the bogus NULL entry too.
+      auto it = subscriptions.find(request[1].str());
+      if (it == subscriptions.end()) {
+         coreutils::Log(coreutils::LOG_DEBUG_1) << "Unknown subscription id: " << request[1].str();
+         return 0;
+      }
+      auto subscription = it->second;
 
       if (request[1].equals(subscription->id)) {
          if (request[0] == "unpublish") {