multimedia/assimp: unbreak build with clang 4.0

code/D3MFImporter.cpp:230:29: error: invalid operands to binary expression ('float (*)(const char *, const char *)' and 'nullptr_t')

vertex.z = ai_strtof>(xmlReader->getAttributeValue(D3MF::XmlTag::z.c_str()), nullptr);
           ~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

PR: 216160
Approved by: Yuri Victorovich <yuri@rawbw.com> (maintainer)
MFH: 2017Q1 (changes behavior)