Reduce vad-whisper-c-api example code.

[whyb]

Reduce vad-whisper-c-api.c example code.

The current code uses a repeated while loop code (two sections of while) to process the tail data.

while(..) {
}
while(..) {
}

And the optimized early calculation of total_size can circulate less each calculation amount and improve efficiency.

So I reduce sherpa-onnx/c-api-examples/vad-whisper-c-api.c some codes.

This change is exactly the same as these PRs: #1510 and #1742

[csukuangfj]

Thank you for your contribution!