[HUST CSE]Fixed an error where parameters could not be assigned

samples/webclient_get_sample.c

@@ -101,6 +101,7 @@ __exit:
     if (session)
     {
         webclient_close(session);
+        session = RT_NULL;
     }
 
     if (buffer)

samples/webclient_post_sample.c

@@ -77,6 +77,7 @@ __exit:
     if (session)
     {
         webclient_close(session);
+        session = RT_NULL;
     }
 
     if (buffer)

samples/webclient_shard_download_sample.c

@@ -132,6 +132,7 @@ __exit:
     if (session)
     {
         webclient_close(session);
+        session = RT_NULL;
     }
 
     return result;

src/webclient_file.c

@@ -132,6 +132,7 @@ __exit:
     if (session != RT_NULL)
     {
         webclient_close(session);
+        session = RT_NULL;
     }
 
     if (ptr != RT_NULL)
@@ -287,6 +288,7 @@ __exit:
     if (session != RT_NULL)
     {
         webclient_close(session);
+        session = RT_NULL;
     }
 
     if (buffer != RT_NULL)