x11-toolkits/fltk: unbreak with clang 4.0

Fl_Tree_Item.cxx:552:38: error: ordered comparison between pointer and zero ('Fl_Tree_Item *' and 'int')

if ( from_parent->deparent(from) < 0 )      // deparent self from current parent
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^ ~

PR: 216046
Obtained from: upstream (branch-1.3)
Approved by: portmgr blanket
MFH: 2017Q1 (changes behavior)