whisper : use correct seek_end when offset is used (ggerganov#833)

Whenever an `offset_ms` is provided, the value of `seek_end` is
calculated incorrectly. This causes Whisper to keep transcribing
after the end of the file.

The current behavior looks like
```
[00:34:40.000 --> 00:34:47.000]   This is an example audio file.
[00:34:47.000 --> 00:34:49.000]   The text has been redacted
[00:34:49.000 --> 00:34:51.000]   This is the end of the audio.
[00:34:51.000 --> 00:34:52.000]   ***
[00:34:52.000 --> 00:34:53.000]   ***
[00:34:53.000 --> 00:34:54.000]   ***
[00:34:55.000 --> 00:34:56.000]   ***
...
```

The expected behavior should be
```
[00:34:40.000 --> 00:34:47.000]   This is an example audio file.
[00:34:47.000 --> 00:34:49.000]   The text has been redacted
[00:34:49.000 --> 00:34:51.000]   This is the end of the audio.
- end of program -
```

This commit changes the calculation of the `seek_end` variable to
only add `seek_start` if a custom `duration_ms` is provided.
Otherwise, it defaults to the end of the file.

Signed-off-by: Thijs Raymakers <thijs@raymakers.nl>

whisper.cpp

@@ -3855,7 +3855,7 @@ int whisper_full_with_state(
     }
 
     const int seek_start = params.offset_ms/10;
-    const int seek_end = seek_start + (params.duration_ms == 0 ? whisper_n_len_from_state(state) : params.duration_ms/10);
+    const int seek_end = params.duration_ms == 0 ? whisper_n_len_from_state(state) : seek_start + params.duration_ms/10;
 
     // if length of spectrogram is less than 1s (100 samples), then return
     // basically don't process anything that is less than 1s