Fix uninitialized exp_n_audio_ctx

[finnvoor]

We've been using whisper.cpp (whisper.spm) in www.detail.co and have run into a number of crashes at this assertion:

whisper.cpp/ggml.c

Lines 1653 to 1657 in 0d22916

	if (ctx->scratch.offs + size_needed > ctx->scratch.size) {
	GGML_PRINT("%s: not enough space in the scratch memory\n", __func__);
	assert(false);
	return NULL;
	}

Debugging these it seems like exp_n_audio_ctx is not initialized before whisper_lang_auto_detect -> whisper_encode, and n_ctx is therefore set to a large random number, creating a mel tensor that runs out of memory.

whisper.cpp/whisper.cpp

Line 1368 in 0d22916

const int n_ctx = wctx.exp_n_audio_ctx > 0 ? wctx.exp_n_audio_ctx : hparams.n_audio_ctx;

Initializing exp_n_audio_ctx to 0 fixes this issue for us.

[jordibruin]

Thanks for finding this @finnvoor