fix std::weak_ptr locking and expiry checks

You're supposed to .lock() instead of TOCTOU checking, of course.
Not sure what I was thinking when I built that. .lock() returns a
default constructed std::shared_ptr on error, which is `false` via
`operator bool`.

src/TPPSMonitor.cpp

@@ -55,8 +55,8 @@ void TPPSMonitor::operator()() {
             std::shared_ptr<TClient> c;
             {
                 ReadLock Lock(mServer.GetClientMutex());
-                if (!ClientPtr.expired()) {
-                    c = ClientPtr.lock();
+                if (auto Locked = ClientPtr.lock()) {
+                    c = std::move(Locked);
                 } else
                     return true;
             }