client: update state earlier to prevent resetTransport calls from killing good transports (#2862)

clientconn.go

@@ -735,6 +735,9 @@ func (ac *addrConn) connect() error {
 		ac.mu.Unlock()
 		return nil
 	}
+	// Update connectivity state within the lock to prevent subsequent or
+	// concurrent calls from resetting the transport more than once.
+	ac.updateConnectivityState(connectivity.Connecting)
 	ac.mu.Unlock()
 
 	// Start a goroutine connecting to the server asynchronously.