pfsync: fix use of invalidated stack variable

Calls to pfsync_send_plus() pass pointers to stack variables.
If pfsync_sendout() then fails it retains the pointer to these stack
variables, accesing them later.

Allocate a buffer and copy the data instead, so that we can retain the
pointer safely.

Reported by: CI KASAN, markj
MFC after: 1 week