server: Calculate remaining data size after a closure is processed

When processing a closure, data in the connection can be consumed again
if the closure itself invokes extra event dispatch. In that case the
remaining data size is also altered, so the variable len should be
updated after the closure is processed.

Signed-off-by: Jaeyoon Jung <jaeyoon.jung@lge.com>
Reviewed-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: Derek Foreman <derekf@osg.samsung.com>