ktls: Check for a NULL send tag in ktls_cleanup()

When using ifnet ktls, and when ktls_reset_send_tag()
fails to allocate a replacement tag, it leaves
the tls session's snd_tag pointer NULL. ktls_cleanup()
tries to release the send tag, and will trip over
this NULL pointer and panic unless NULL is checked for.

Reviewed by: jhb
Sponsored by: Netflix