Fixed njs_vm_handle_events().

Fixed njs_vm_handle_events().

Fixed typo introduced in 61bf7a31e685. Event loop is expected to process two
queues: posted events and promise events according to the spec. The "do while"
loop was introduced to handle possible promise events which may appear as
a result of posted events processing. The loop termination condition should
check for emptyness of promise events queue, because posted events can only be
created outside the loop (externally).