Keep track of iterator end in `_nextv()` fallback

[vweevers]

Currently, nextv() can only signal the natural end of an iterator by yielding an empty array. In the fallback _nextv() implementation below, when _next() signals end by yielding an undefined key and/or value (line 147), this means _nextv() does not signal end (line 148, unless acc is empty). Consequently, a consumer will call nextv() again, which means we call _next() again (line 160) which has already signaled end.

Whether that's a problem depends on implementation. It is in many-level (the replacement for multileveldown that I'm working on atm, and can easily have its own fix) and could be in others too. It's not a problem for implementations written so far (memory-level, classic-level, browser-level) because those implement their own optimized _nextv().

Solution: set a new private kEnded property to true on line 148, and check that it's false before calling _next(). A seek() should reset it to false. Maybe use kEnded in more places, as a general protection.

abstract-level/abstract-iterator.js

Lines 142 to 161 in 601b47c

	_nextv (size, options, callback) {
	const acc = []
	const onnext = (err, key, value) => {
	if (err) {
	return callback(err)
	} else if (this[kLegacy] ? key === undefined && value === undefined : key === undefined) {
	return callback(null, acc)
	}
	acc.push(this[kLegacy] ? [key, value] : key)
	if (acc.length === size) {
	callback(null, acc)
	} else {
	this._next(onnext)
	}
	}
	this._next(onnext)
	}