wg: Avoid leaking mbufs when the input handshake queue is full
Reviewed by: grehan
Sponsored by: The FreeBSD Foundation
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D29011
(cherry picked from commit a11009dccb6a2e75de2b8f1b45a0896eda2e6d85)