优化网络（response为nil，网络异常）

DreamSleep/DreamSleep/Basement/Network/DSNetworkTool.m

@@ -66,7 +66,7 @@ NSString * const NetworkUnableError = @"网络不给力，请检查您的网络
         
         // 处理服务器内部错误
         NSHTTPURLResponse *httpRes = (NSHTTPURLResponse *)response;
-        if (!(httpRes.statusCode == 200)) {
+        if (httpRes && !(httpRes.statusCode == 200)) {
             DSLog(@"httpRes：%@", httpRes);
             NSString *errInfo = [NSHTTPURLResponse localizedStringForStatusCode:httpRes.statusCode];
             failure(@{@"errorCode":@(DSResCodeServerErr), @"errMessage":errInfo});