Fixes #66714: imageconvolution breakage in 5.5.9

[#66714](https://bugs.php.net/bug.php?id=66714)

5.5.9 included some GD fixes related to [#66356](https://bugs.php.net/bug.php?id=66356). One of those fixes changed the 
above section of imageconvolution, but the variable was mistyped.

EDIT: Have confirmed that this does indeed fix the issue after compiling my php-5.5 branch with this fix.