graphics/cal3d: unbreak build with Clang 6 (C++14 by default)

loader.cpp:899:12: error: cannot initialize return object of type 'CalCoreKeyframe *' with an rvalue of type 'bool'

return false;
       ^~~~~

loader.cpp:1448:11: error: no viable conversion from returned value of type 'bool' to function return type 'CalCoreSkeletonPtr' (aka 'RefPtr<CalCoreSkeleton>')

return false;
       ^~~~~

../../src/cal3d/refptr.h:16:9: note: candidate constructor not viable: no known conversion from 'bool' to 'CalCoreSkeleton *' for 1st argument

RefPtr(T* ptr = 0)
^

../../src/cal3d/refptr.h:22:9: note: candidate constructor not viable: no known conversion from 'bool' to 'const RefPtr<CalCoreSkeleton> &' for 1st argument

RefPtr(const RefPtr<T>& ptr)
^

Reported by: pkg-fallout (blocks 5 ports)
Approved by: portmgr blanket