Description:
------------
When using array keys as variables in the list() function  there is a noticiable memory leak.

It doesn't seem to matter if you assign values to the array key's before using them in the array.



Reproduce code:
---------------
//memory leak!
while (1)
{
 $out = array(); 
 $arr = array('a','b','c');
 list($out['a'], $out['b'], $out['c']) = $arr;
}

//NO memory leak!!
while (1)
{
 $out = array(); 
   $a = &$out['a'];
   $b = &$out['b'];
   $c = &$out['c'];

 $arr = array('a','b','c');
 list($a, $b, $c) = $arr;
}

Expected result:
----------------
Output every 4000 loops
#  | Memory usage

4000  |  68 KB
8000  |  68 KB
12000  |  68 KB
16000  |  68 KB
20000  |  68 KB
24000  |  68 KB
28000  |  68 KB
32000  |  68 KB
36000  |  68 KB
40000  |  68 KB
44000  |  68 KB
48000  |  68 KB
52000  |  68 KB
56000  |  68 KB
60000  |  68 KB
64000  |  68 KB
68000  |  68 KB
72000  |  68 KB
76000  |  68 KB
80000  |  68 KB
84000  |  68 KB
88000  |  68 KB
92000  |  68 KB
96000  |  68 KB
100000  |  68 KB
104000  |  68 KB
108000  |  68 KB
112000  |  68 KB




Actual result:
--------------
Output  every 4000 loops
# | Memory usage

4000  |  349 KB
8000  |  630 KB
12000  |  911 KB
16000  |  1.2 MB
20000  |  1.4 MB
24000  |  1.7 MB
28000  |  2.0 MB
32000  |  2.3 MB
36000  |  2.5 MB
40000  |  2.8 MB
44000  |  3.1 MB
48000  |  3.4 MB
52000  |  3.6 MB
56000  |  3.9 MB
60000  |  4.2 MB
64000  |  4.5 MB
68000  |  4.7 MB
72000  |  5.0 MB
76000  |  5.3 MB
80000  |  5.6 MB
84000  |  5.8 MB
88000  |  6.1 MB
92000  |  6.4 MB
96000  |  6.7 MB
100000  |  6.9 MB
104000  |  7.2 MB
108000  |  7.5 MB
112000  |  7.8 MB