FWIW: Here is what official docs say for the return value of CMSampleBufferGetImageBuffer:
"Result is a CVImageBuffer of media data. The result will be NULL if the CMSampleBuffer does not contain a CVImageBuffer, or if the CMSampleBuffer contains a CMBlockBuffer, or if there is some other error."
Also note that the caller does not own the returned dataBuffer from CMSampleBufferGetImageBuffer, and must retain it explicitly if the caller needs to maintain a reference to it.
Hopefully this info helps.