fixed deadlock on Stop when in connecting state (aspnet#38)

Author: gonzo-coder

src/signalrclient/connection_impl.cpp

@@ -148,6 +148,7 @@ namespace signalr
         if (redirect_count >= MAX_NEGOTIATE_REDIRECTS)
         {
             change_state(connection_state::disconnected);
+            m_start_completed_event.cancel();
             callback(std::make_exception_ptr(signalr_exception("Negotiate redirection limit exceeded.")));
             return;
         }
@@ -178,13 +179,15 @@ namespace signalr
                             .append(e.what()));
                     }
                     connection->change_state(connection_state::disconnected);
+                    connection->m_start_completed_event.cancel();
                     callback(exception);
                     return;
                 }
 
                 if (!response.error.empty())
                 {
                     connection->change_state(connection_state::disconnected);
+                    connection->m_start_completed_event.cancel();
                     callback(std::make_exception_ptr(signalr_exception(response.error)));
                     return;
                 }
@@ -219,6 +222,7 @@ namespace signalr
                 if (!foundWebsockets)
                 {
                     connection->change_state(connection_state::disconnected);
+                    connection->m_start_completed_event.cancel();
                     callback(std::make_exception_ptr(signalr_exception("The server does not support WebSockets which is currently the only transport supported by this client.")));
                     return;
                 }