Fixes the find-next-loop when deleting the last message. The reverse
loop with store.Prev() will break too early because the value of
'previous' was not reset correctly.
Fixes: d941960f "delete: improve find next function"
References: https://todo.sr.ht/~rjarry/aerc/59
Signed-off-by: Koni Marti <koni.marti@gmail.com>
Tested-by: Tim Culverhouse <tim@timculverhouse.com>