Fix uninitialised variable in two Windows event loops.

Fix uninitialised variable in two Windows event loops.

If (Msg)WaitForMultipleObjects returns WAIT_TIMEOUT, we expect 'next'
to have been initialised. This can occur without having called
run_timers(), if a toplevel callback was pending, so we can't expect
run_timers to have reliably initialised 'next'.

I'm not actually convinced this could have come up in either of the
affected programs (Windows PSFTP and Plink), due to the list of things
toplevel callbacks are currently used for, but it certainly wants
fixing anyway for the future.

Spotted by Coverity.