Do not clear pxEndPoint in TCPReturnPacket()

[htibosch]

Description

In response to issue #1077 :

@XIAO-Q7, thank you for reporting the issue and for the research that you did. Also thanks to @evpopov who encountered the same problem.

I managed to reproduce the problem if I use the following configuration:

• DUT has IP-address 192.168.2.127
• DUT has no gateway (0.0.0.0)
• The TCP remote party has IP address 172.16.0.1

DUT tries to connect to remote device, but eARPGetCacheEntry() clears the endpoint, which leads to a dereference of NULL.

Here is a summary of the proposed change:

-     eResult = eARPGetCacheEntry( &ulDestinationIPAddress, &xMACAddress,
-                                  &( pxNetworkBuffer->pxEndPoint ) );
+     eResult = eARPGetCacheEntry( &ulDestinationIPAddress, &xMACAddress,
+                                  &pxEndPoint );

     if( eResult == eARPCacheHit )
     {
         pvCopySource = &xMACAddress;
+        pxNetworkBuffer->pxEndPoint = pxEndPoint;
     }
     else
     {
         pvCopySource = &pxEthernetHeader->xSourceAddress;
     }

+    if( pxNetworkBuffer->pxEndPoint == NULL )
+    {
+        break;
+    }

I hadn't had the time to check the code coverage yet.

Test Steps

Use a similar configuration and try to connect to the remote device.

Checklist:

• I have tested my changes. No regression in existing tests.
• I have modified and/or added unit-tests to cover the code changes in this Pull Request.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[tony-josi-aws]

/bot run formatting

[shubnil]

Initially I had some confusion whether eARPGetCacheEntry will return a eARPCacheHit or eARPCacheMiss in this schenario and if the check (pxNetworkBuffer->pxEndPoint == NULL ) needs to be inside the return value check or outside as it is now. However, on some more exploration and internal discussions, it is clear that eARPGetCacheEntry will return eARPCacheMiss and the check can be outside and it will not have any other side-effect. With this I am good with the change.