socket: Check for a NULL ctloutput handler in the SO_SETFIB handler

[markj]

Summary

Some protocols, e.g., ng_btsocket_l2cap_raw_protosw, do not have one.

Reported by: syzkaller
Fixes: caccbaef8e26 ("socket: Move SO_SETFIB handling to protocol layers")
MFC after: 1 week

[glebius]

I would do it differently. In uipc_domain.c add pr_ctloutput_notsupp() and add it with NOTSUPP() macro to pr_init() in the same file.