core/reactor: Fix use-after-free on io_event's promise
core/reactor: Fix use-after-free on io_event's promise

reactor::submit_io() tries accesss pr after calling
flush_pending_io(), which may free the promise if it fails to submit
the IO due to EBADF.

Fixes #175.

Message-Id: <e64da885b1a6cc2f4f5b7aa53afb796f44403b24@scylladb.com>

dreamsxin/seastar,dreamsxin/seastar,avikivity/seastar,scylladb/seastar,dreamsxin/seastar,kjniemi/seastar,kjniemi/seastar,avikivity/seastar,syuu1228/seastar,cloudius-systems/seastar,avikivity/seastar,syuu1228/seastar,scylladb/seastar,syuu1228/seastar,scylladb/seastar,cloudius-systems/seastar,cloudius-systems/seastar,kjniemi/seastar