vinyl: fix merge iterator
vinyl: fix merge iterator

It doesn't work if there are more than 2 sources. The fix is
self-obvious.

Closes #1725

mejedi/tarantool,mejedi/tarantool,rtsisyk/tarantool,rtsisyk/tarantool,rtsisyk/tarantool,rtsisyk/tarantool,mejedi/tarantool,mejedi/tarantool