MFH: r432965

textproc/htmldoc: unbreak with clang 4.0

htmlsep.cxx:527:19: error: ordered comparison between pointer and zero ('int *' and 'int')

if (heading >= 0)
    ~~~~~~~ ^  ~

PR: 216159
Reported by: antoine (via exp-run)
Approved by: portmgr blanket
Approved by: ports-secteam blanket