Description:
------------
If an ImagickPixelIterator is dereferenced directly without storing a ref to a variable first, the process will crash when calling methods on an ImagickPixel of a row we are iterating over. Any ImagickPixel method will trigger the "wand->signature == WandSignature" assertion, each on a different line in wand/pixel-wand.c.

Tested on PHP 7.0.14 with imagick ext 3.4.2 (ImageMagick 6.9.7-0 Q16 x86_64 2016-12-18). PHP 5.x is affected as well.

Test script:
---------------
$im = new Imagick();
$im->newImage(1, 1, 'black');

// This works fine
$it = $im->getPixelIterator();
$row = $it->getCurrentIteratorRow();
$rgb = $row[0]->getColor();

var_dump($rgb);

// This crashes with SIGABRT
$row = $im->getPixelIterator()->getCurrentIteratorRow();
$rgb = $row[0]->getColor();

var_dump($rgb);

Expected result:
----------------
array(4) {
  ["r"]=>
  int(0)
  ["g"]=>
  int(0)
  ["b"]=>
  int(0)
  ["a"]=>
  int(1)
}
array(4) {
  ["r"]=>
  int(0)
  ["g"]=>
  int(0)
  ["b"]=>
  int(0)
  ["a"]=>
  int(1)
}

Actual result:
--------------
array(4) {
  ["r"]=>
  int(0)
  ["g"]=>
  int(0)
  ["b"]=>
  int(0)
  ["a"]=>
  int(1)
}
Assertion failed: (wand->signature == WandSignature), function PixelGetRed, file wand/pixel-wand.c, line 1338.
Abort trap: 6