Description:
------------
imagecreatefromstring seems to segfault when a a bogus string with letters is provided, works when this string is only non-char.

Reproduce code:
---------------
27/05 12:15 root@meteor /home/julien/www# php                                                                                                    
<?php
  $test = str_repeat("/", 10);
  $gd1 = imagecreatefromstring($test);
  echo "OK 10\n";
?>

Warning: imagecreatefromstring() [http://www.php.net/function.imagecreatefromstring]: Data is not in a recognized format. in - on line 3
OK 10

27/05 12:15 root@meteor /home/julien/www# php                                                                                                      
<?php
  $test = str_repeat("bad", 5);
  $gd1 = imagecreatefromstring($test);
  echo "Normal is OK \n";
?>
zsh: segmentation fault  php

27/05 12:15 root@meteor /home/julien/www# php                                                                                                    
<?php
  $test = str_repeat("/", 10);
  $gd1 = imagecreatefromstring($test);
  echo "OK 10\n";
?>

Warning: imagecreatefromstring() [http://www.php.net/function.imagecreatefromstring]: Data is not in a recognized format. in - on line 3
OK 10

27/05 12:15 root@meteor /home/julien/www# php                                                                                                      
<?php
  $test = str_repeat("a", 10);
  $gd1 = imagecreatefromstring($test);
  echo "OK 10\n";
?>
zsh: segmentation fault  php

27/05 12:15 root@meteor /home/julien/www# php                                                                                                    
<?php
  $test = str_repeat("*", 10);
  $gd1 = imagecreatefromstring($test);
  echo "OK 10\n";
?>

Warning: imagecreatefromstring() [http://www.php.net/function.imagecreatefromstring]: Data is not in a recognized format. in - on line 3
OK 10

27/05 12:16 root@meteor /home/julien/www# php                                                                                                      
<?php
  $test = str_repeat("q", 10);
  $gd1 = imagecreatefromstring($test);
  echo "OK 10\n";
?>
zsh: segmentation fault  php


Expected result:
----------------
No segfaults, just warnings.

Actual result:
--------------
Segfaults.