Fix potential leak in TCPEndPoint::LwIPHandleDataReceived (#11447)

#### Problem

`TCPEndPoint::LwIPHandleDataReceived()` receives a packet buffer and,
in the normal case, forwards its ownership via `PostEvent()`. If that
fails, `LwIPHandleDataReceived()` is responsible for freeing the pbuf,
but doesn't.

From LwIP documentation, “If the callback function returns ERR_OK or
ERR_ABRT it must have freed the pbuf”.

#### Change overview

Call `pbuf_free()` if `PostEvent()` fails.

#### Testing

None; we don't have any way to instrument LwIP to verify this.

Author: kpschoedel

src/inet/TCPEndPoint.cpp

@@ -1086,7 +1086,13 @@ err_t TCPEndPoint::LwIPHandleDataReceived(void * arg, struct tcp_pcb * tpcb, str
         res = ERR_ABRT;
 
     if (res != ERR_OK)
+    {
+        if (p != nullptr)
+        {
+            pbuf_free(p);
+        }
         tcp_abort(tpcb);
+    }
 
     return res;
 }