NetworkPkg: Fixes to static code analysis hits

Introducing fixes to memory leak issues identified by static code analysis
tool.

Cc: Jiaxin Wu <jiaxin.wu@intel.com>
Cc: Siyuan Fu <siyuan.fu@intel.com>
Signed-off-by: Maciej Rabeda <maciej.rabeda@linux.intel.com>
Reviewed-by: Philippe Mathieu-Daude <philmd@redhat.com>
Reviewed-by: Siyuan Fu <siyuan.fu@intel.com>
Reviewed-by: Jiaxin Wu <jiaxin.wu@intel.com>

NetworkPkg/Dhcp6Dxe/Dhcp6Io.c

@@ -203,6 +203,7 @@ Dhcp6EnqueueRetry (
     //
     // Unexpected message type.
     //
+    FreePool(TxCb);
     return EFI_DEVICE_ERROR;
   }