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

(cherry picked from commit 81debbd60e5773e812e9227a2003ea88699580be)