vad : fix buffer overflow in sample reduction loop

[josephsellers]

Summary

Fix heap corruption crash in VAD processing caused by buffer overflow in the sample reduction loop.

Problem

The buffer size calculation loop (line ~6661) uses n_samples - 1 as the upper bound:

segment_end_samples = std::min(segment_end_samples, n_samples - 1);
filtered_n_samples  += (segment_end_samples - segment_start_samples);

But the copy loop (line 6696) uses n_samples:

segment_end_samples = std::min(segment_end_samples, n_samples);  // BUG: inconsistent bound
int segment_length = segment_end_samples - segment_start_samples;

This allows segment_length to be up to 1 sample larger per segment than what was allocated, causing writes past the end of filtered_samples buffer.

Symptom

• malloc(): corrupted top size
• malloc(): invalid size (unsorted)
• Crashes after VAD completes: whisper_vad: Reduced audio from X to Y samples
• Intermittent (depends on heap layout)

Fix

Use consistent bounds (n_samples - 1) in both loops.

Fixes #3403

[danbev]

@josephsellers Thanks!