Use the handle.defined() method to avoid dereferencing past-the end iter
Use the handle.defined() method to avoid dereferencing past-the end iter

Previous the lookup call would not fire the assert b/c it's a release
build and would dereference and return some value from a past-the-end
iterator which would then luckily be null. Switching to use the defined
method checks if the handle is actually available before doing the
lookup.

wilsonCernWq/ospray,ospray/OSPRay,ospray/OSPRay,ospray/OSPRay,wilsonCernWq/ospray,ospray/OSPRay,wilsonCernWq/ospray,MengjiaoH/ospray,MengjiaoH/ospray,MengjiaoH/ospray,MengjiaoH/ospray,MengjiaoH/ospray