bpf: Fix the write filter for detached descriptors

A BPF descriptor only has an associated interface descriptor once it is
attached to an interface, e.g., with BIOCSETIF. Avoid dereferencing a
NULL pointer in filt_bpfwrite() if the BPF descriptor is not attached.

Reviewed by: ae
Reported by: syzbot+ae45d5166afe15a5a21d@syzkaller.appspotmail.com
Fixes: ded77e0237a8 ("Allow the BPF to be select for write.")
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D32561