whisper : fix int overflow in whisper_full_parallel chunk offsets

[kmadiar]

Fixes #4039.

whisper_full_parallel() computed the per-chunk time offset as 100 * ((i + 1) * n_samples_per_processor) / WHISPER_SAMPLE_RATE in int before adding it to the int64_t offset_t. For chunk boundaries later than ~22 min at 16 kHz (100 * samples > INT_MAX) the multiplication overflows, corrupting the merged segment t0/t1 and the split times printed in the log.

This PR computes the offset once per chunk in int64_t (100LL * ...) and reuses it for both the segment merge and the split-time log.

Verification — 46 min file (samples/jfk.wav looped), --processors 2, boundary at 23:00:

Before, built with -fsanitize=undefined:

src/whisper.cpp:7980:30: runtime error: signed integer overflow: 100 * 22080000 cannot be represented in type 'int'
src/whisper.cpp:7981:30: runtime error: signed integer overflow: 100 * 22080000 cannot be represented in type 'int'
src/whisper.cpp:8023:9:  runtime error: signed integer overflow: 100 * 22080000 cannot be represented in type 'int'
whisper_full_parallel: split 1 - 00:-21:-44.-350

After: no overflow reports, split 1 - 00:23:00.000, all segment offsets in the JSON output non-negative and monotonic. --processors 4 (Release) reports splits at 00:11:30 / 00:23:00 / 00:34:30.

Note: on Apple clang Release builds the UB happens to yield the correct value, which is why this is mostly seen on MSVC/Windows (as in the issue); the UBSan build reproduces it on macOS.

I have a small regression test (whisper_full_parallel on a 45 min silent buffer with duration_ms = 1000, checking the logged split time via whisper_log_set; ~3 s, fails on master with 00:-22:-14.-350). Happy to add it to this PR or a follow-up if you'd like it.

Token-level timestamps in parallel mode (#2036, #3726) are a separate issue and unchanged here.