SDK / exoplayer

Discovered while investigating Issue: google/ExoPlayer#10823

Example stack trace with the previous code (I added the index value for
debugging):

```
playerFailed [eventTime=44.07, mediaPos=44.01, window=0, period=0, errorCode=ERROR_CODE_FAILED_RUNTIME_CHECK
  androidx.media3.exoplayer.ExoPlaybackException: Unexpected runtime error
      at androidx.media3.exoplayer.ExoPlayerImplInternal.handleMessage(ExoPlayerImplInternal.java:635)
      at android.os.Handler.dispatchMessage(Handler.java:102)
      at android.os.Looper.loopOnce(Looper.java:202)
      at android.os.Looper.loop(Looper.java:291)
      at android.os.HandlerThread.run(HandlerThread.java:67)
  Caused by: java.lang.IllegalArgumentException: index=-1
      at androidx.media3.common.util.Assertions.checkArgument(Assertions.java:55)
      at androidx.media3.extractor.text.webvtt.WebvttSubtitle.getEventTime(WebvttSubtitle.java:62)
      at androidx.media3.extractor.text.SubtitleOutputBuffer.getEventTime(SubtitleOutputBuffer.java:56)
      at androidx.media3.exoplayer.text.TextRenderer.getCurrentEventTimeUs(TextRenderer.java:435)
      at androidx.media3.exoplayer.text.TextRenderer.render(TextRenderer.java:268)
      at androidx.media3.exoplayer.ExoPlayerImplInternal.doSomeWork(ExoPlayerImplInternal.java:1008)
      at androidx.media3.exoplayer.ExoPlayerImplInternal.handleMessage(ExoPlayerImplInternal.java:509)
      at android.os.Handler.dispatchMessage(Handler.java:102)
      at android.os.Looper.loopOnce(Looper.java:202)
      at android.os.Looper.loop(Looper.java:291)
      at android.os.HandlerThread.run(HandlerThread.java:67)
]
```

#minor-release

PiperOrigin-RevId: 492464180
(cherry picked from commit 33bbb951)