renderer.cpp: check for null pointers

Fixes Coverity CID 709292 and 709293:
Dereference after null check (FORWARD_NULL) [select defect]
1917    if (original == NULL || clip == NULL) {
1918        kDebug() << "// ERROR GRABBING CLIP AFTER SPLIT";
1919    }
1920    Mlt::Service clipService(original->get_service());
1921    Mlt::Service dupService(clip->get_service());
1922    delete original;
1923    delete clip;