Fix NPE in NettyAsyncHttpProvider.toString()

When maxConnectionsTotal isn't set, freeConnections is always null.

Author: Kelly Byrd

src/main/java/com/ning/http/client/providers/netty/NettyAsyncHttpProvider.java

@@ -271,8 +271,9 @@ public NettyAsyncHttpProvider(AsyncHttpClientConfig config) {
 
     @Override
     public String toString() {
+        int availablePermits = freeConnections != null ? freeConnections.availablePermits() : 0;
         return String.format("NettyAsyncHttpProvider:\n\t- maxConnections: %d\n\t- openChannels: %s\n\t- connectionPools: %s",//
-                config.getMaxTotalConnections() - freeConnections.availablePermits(),//
+                config.getMaxTotalConnections() - availablePermits,//
                 openChannels.toString(),//
                 connectionsPool.toString());
     }