Description:
------------
Using $array[] for reading is permitted. But reference &$array[] erroneously is allowed and appends NULL element silently.

Reproduced in all PHP versions up to latest 7.0.0rc1

https://3v4l.org/0l8OE

Test script:
---------------
function bar(&$var) {}

$foo = [10, 20];
$ref = &$foo[];
bar($foo[]);
var_dump($foo);

Expected result:
----------------
Fatal error: Cannot use [] for reading in ...

Actual result:
--------------
array(4) {
  [0]=>
  int(10)
  [1]=>
  int(20)
  [2]=>
  &NULL
  [3]=>
  NULL
}