Null pointer deref with no cpu backend loaded.

[Lucius-Q-User]

https://github.com/ggerganov/whisper.cpp/blob/master/src/whisper.cpp#L1358 unconditionally adds the cpu backend, which might not exist if it is dynamically linked. This results in a nullptr being added to that array, and a crash later. (Forgetting to load the cpu backend probably was skill issue on my part, but it would be nice to have a clear error instead of a crash)