MT#55283 Set valid decrypt fields for non-forwarding kernel targets

In case of the reverse direction of a subscription, no stream handler will be found. Previously, the
decrypt struct was left zeroed, the kernel module rejected this with ``EINVAL`` and the daemon logged:

```
[core] Failed to push relay stream to kernel: Invalid argument
```

This patch sets ``REC_NULL``/``REH_NULL`` and marks the target as non-forwarding, matching what is
expected by ``validate_srtp``.

Closes #2126

Change-Id: I5aa2b490c65e5d97dc9b2ddd24c09f9ae7b63892
(cherry picked from commit 47371520ee)
(cherry picked from commit d435a153bf)

daemon/media_socket.c

@@ -1637,6 +1637,9 @@ static const char *kernelize_target(kernelize_state *s, struct packet_stream *st
 		// nothing to forward
 		s->non_forwarding = true;
 		s->blackhole = true;
+		reti->non_forwarding = 1;
+		reti->decrypt.cipher = REC_NULL;
+		reti->decrypt.hmac = REH_NULL;
 		return NULL;
 	}