chtls: Added a check to avoid NULL pointer dereference

In case of server removal lookup_stid() may return NULL pointer, which
is used as listen_ctx. So added a check before accessing this pointer.

Fixes: cc35c88a ("crypto : chtls - CPL handler definition")
Signed-off-by: Vinay Kumar Yadav <vinay.yadav@chelsio.com>
Signed-off-by: Ayush Sawal <ayush.sawal@chelsio.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>