converters/wkhtmltopdf: unbreak with clang 4.0 (same as r431742)

src/3rdparty/webkit/Source/WebCore/html/HTMLImageElement.cpp:77:24: error: ordered comparison between pointer and zero ('const int *' and 'int')

if (optionalHeight > 0)
    ~~~~~~~~~~~~~~ ^ ~

PR: 216211
Obtained from: upstream (excerpt)
Approved by: portmgr blanket