[Agent] Apache http client plugin bug (#1448)

* #1447

Fixed the after method interceptor for the apache http client NPE error when the response is null cause by the doExecute method throw a exception.

apm-sniffer/apm-sdk-plugin/httpClient-4.x-plugin/src/main/java/org/apache/skywalking/apm/plugin/httpClient/v4/HttpClientExecuteInterceptor.java

@@ -18,23 +18,14 @@
 
 package org.apache.skywalking.apm.plugin.httpClient.v4;
 
-import org.apache.http.HttpHost;
-import org.apache.http.HttpRequest;
-import org.apache.http.HttpResponse;
-import org.apache.http.StatusLine;
-import org.apache.skywalking.apm.agent.core.context.CarrierItem;
-import org.apache.skywalking.apm.agent.core.context.ContextCarrier;
-import org.apache.skywalking.apm.agent.core.context.ContextManager;
-import org.apache.skywalking.apm.agent.core.context.tag.Tags;
-import org.apache.skywalking.apm.agent.core.context.trace.AbstractSpan;
-import org.apache.skywalking.apm.agent.core.context.trace.SpanLayer;
-import org.apache.skywalking.apm.agent.core.plugin.interceptor.enhance.EnhancedInstance;
-import org.apache.skywalking.apm.agent.core.plugin.interceptor.enhance.InstanceMethodsAroundInterceptor;
-import org.apache.skywalking.apm.agent.core.plugin.interceptor.enhance.MethodInterceptResult;
-import org.apache.skywalking.apm.network.trace.component.ComponentsDefine;
-
 import java.lang.reflect.Method;
 import java.net.URL;
+import org.apache.http.*;
+import org.apache.skywalking.apm.agent.core.context.*;
+import org.apache.skywalking.apm.agent.core.context.tag.Tags;
+import org.apache.skywalking.apm.agent.core.context.trace.*;
+import org.apache.skywalking.apm.agent.core.plugin.interceptor.enhance.*;
+import org.apache.skywalking.apm.network.trace.component.ComponentsDefine;
 
 public class HttpClientExecuteInterceptor implements InstanceMethodsAroundInterceptor {
 
@@ -73,6 +64,7 @@ public class HttpClientExecuteInterceptor implements InstanceMethodsAroundInterc
             return ret;
         }
 
+        if (ret != null) {
             HttpResponse response = (HttpResponse)ret;
             StatusLine responseStatusLine = response.getStatusLine();
             if (responseStatusLine != null) {
@@ -83,6 +75,8 @@ public class HttpClientExecuteInterceptor implements InstanceMethodsAroundInterc
                     Tags.STATUS_CODE.set(span, Integer.toString(statusCode));
                 }
             }
+        }
+        
         ContextManager.stopSpan();
         return ret;
     }