Fix concat_function use-after-free on out-of-memory error

[iluuu1994]

Introduced by GH-10049

Unfortunately, I was not able to provide a test case for this. The example from 727e26f#commitcomment-114423406 only worked in the very specific case of keeping the file in the project root, and running it without run-tests.php.

[ndossche]

Thanks for debugging and fixing this

[iluuu1994]

Actually, this worked:

--TEST--
Bug #79836 (Segfault in concat_function)
--INI--
memory_limit=10M
--FILE--
<?php
class Foo {
    public function __toString() {
        return str_repeat('a', 10);
    }
}

$i = str_repeat('a', 5 * 1024 * 1024);
$e = new Foo();
$e .= $i;
?>
--EXPECT--

I will amend this test.