Description:
------------
I've found a memory leak that occurs when iterating over a generator that directly yields from another generator inside a loop (see genLeak()).

It seems that this can be worked around by assigning the result of the yield from to a temp var and then yielding that var (see genNoLeak()).

Directly yielding is also fine (see gen()).

Test script:
---------------
<?php

function foo()
{
    yield array_fill(0, 10000, 4);
}

function gen()
{
    while(true){
        yield array_fill(0, 10000, 4);
        print memory_get_usage(false).PHP_EOL;
    }
}

function genLeak()
{
    while(true){
        yield from foo();
        print memory_get_usage(false).PHP_EOL;
    }
}

function genNoLeak()
{
    while(true){
        $tmp = yield from foo();
        yield $tmp;
        print memory_get_usage(false).PHP_EOL;
    }
}

foreach(genLeak() as $i){}


Expected result:
----------------
Memory usage remains constant though each iteration of the loop.

Actual result:
--------------
Memory usage rises until php runs out of memory and crashes.