unix: Fix a missing initialization in uipc_sosend_stream_or_seqpacket()

This could be triggered by an in-kernel sender, of which I can't find
any examples.

Fixes: d15792780760 ("unix: new implementation of unix/stream & unix/seqpacket")
Reviewed by: glebius
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D58673

(cherry picked from commit 6dfd710963da7fe6086bcfb705b32b6a3646848f)