whisper : default-initialize whisper_mel to avoid uninitialized read

[ousamabenyounes]

What

whisper_full() / whisper_full_with_state() called with n_samples == 0 on a
freshly allocated state can dereference a NULL mel buffer inside the encoder, or
silently return 0 — the outcome depends on heap garbage, so the crash is
intermittent.

Give struct whisper_mel default member initializers so a never-computed mel
reads as "0 frames".

Why

The mel spectrogram is only computed when there is audio:
if (n_samples > 0) { ... whisper_pcm_to_mel_with_state(...) }. For
n_samples == 0 the mel is never touched. But whisper_mel had no member
initializers, and the state is allocated with new whisper_state, so
mel.n_len / n_len_org / n_mel were indeterminate heap garbage while mel.data
is a correctly-constructed empty vector (NULL data pointer).

seek_end is derived from mel.n_len_org. If the garbage lands below the
delta_min guard the call quietly returns 0; otherwise decoding proceeds and
whisper_encode_internal copies the mel with garbage dimensions from the empty
(NULL) buffer → read at address 0. The zero-sample input is reachable from real
use (a push-to-talk recording stopped before the first capture buffer yields a
valid-header, zero-frame WAV).

With the fields default-initialized to 0, n_samples == 0 deterministically
takes the existing "input too short" path and returns 0 segments. The two code
paths that legitimately set a mel (log_mel_spectrogram,
whisper_set_mel_with_state) overwrite all three fields before any read, so the
defaults are observable only on the never-computed mel — the bug case.

Test verification (RED → GREEN)

Added tests/test-whisper-zero-samples.cpp: load for-tests-ggml-tiny.bin, call
whisper_full(ctx, params, nullptr, 0), assert rc == 0 and n_segments == 0.

Because the read is of uninitialized memory, the deterministic RED is shown with
valgrind on the unpatched struct (only the production change reverted, the new
test applied):

RED (upstream base — whisper_mel without initializers):

==316== Conditional jump or move depends on uninitialised value(s)
==316==    at 0x4987C4C: whisper_full_with_state (whisper.cpp:6886)
==316==    by 0x498B768: whisper_full (whisper.cpp:7811)
==316==    by 0x109375: main (test-whisper-zero-samples.cpp:30)
==316==  Uninitialised value was created by a heap allocation
==316==    by 0x49785B7: whisper_init_state (whisper.cpp:3388)
...
ERROR SUMMARY: (valgrind exit code 1)

GREEN (with the fix):

test-whisper-zero-samples: OK
==75== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)
valgrind exit: 0

Normal (non-valgrind) run of the new test plus the existing whisper-cli tiny
test both pass.

Fixes #3978

[ousamabenyounes]

Removed the verbose comments as requested. The zero-samples test still passes.
thank you for your review

[danbev]

@ousamabenyounes Thanks! Can you take a look at the conflict?

[ousamabenyounes]

@danbev Rebased onto the latest master to clear the conflict — the PR is now mergeable again (02c45f5b). The change is unchanged from your review: whisper_mel's three fields default-initialized to 0, plus the zero-samples regression test.

Locally, ctest -L gh is green (4/4, including test-whisper-zero-samples) on the rebased base. The GitHub Actions checks are showing action_required (fork-PR workflows waiting on a maintainer approval-to-run) rather than a failure.