stream : add nullptr check of whisper_context (#3283)

stream : add nullptr check of whisper_context (#3283)

* stream : add nullptr check of whisper_context

This commit adds a check to ensure that the `whisper_context` is not
null after initialization.

The motivation for this is that currently, if the initialization fails,
the program continues to run leading to a segmentation fault. This sort
of check is performed by others examples like whisper-cli.

Refs: https://github.com/ggml-org/whisper.cpp/issues/3280#issuecomment-3003778035

* examples : add nullptr check for whisper_context