graphics/nvidia-texture-tools: unbreak build with Clang 6 (C++14 by default)

src/nvimage/ImageIO.cpp:138:10: error: cannot initialize return object of type 'nv::FloatImage *' with an rvalue of type 'bool'

return false;
       ^~~~~

src/nvimage/ImageIO.cpp:236:12: error: cannot initialize return object of type 'nv::Image *' with an rvalue of type 'bool'

return false;
       ^~~~~

src/nvimage/ImageIO.cpp:257:11: error: cannot initialize return object of type 'nv::Image *' with an rvalue of type 'bool'

return false;
       ^~~~~

src/nvimage/ImageIO.cpp:624:10: error: cannot initialize return object of type 'nv::Image *' with an rvalue of type 'bool'

return false;
       ^~~~~

src/nvimage/ImageIO.cpp:632:10: error: cannot initialize return object of type 'nv::Image *' with an rvalue of type 'bool'

return false;
       ^~~~~

src/nvimage/ImageIO.cpp:639:10: error: cannot initialize return object of type 'nv::Image *' with an rvalue of type 'bool'

return false;
       ^~~~~

Reported by: pkg-fallout (blocks 9 ports)
Obtained from: upstream
Approved by: portmgr blanket