Avoid div-by-zero in waveform resampling code
Avoid div-by-zero in waveform resampling code

```csharp
2022-06-26 06:20:32 [error]: An unobserved error has occurred.
2022-06-26 06:20:32 [error]: System.DivideByZeroException: Attempted to divide by zero.
2022-06-26 06:20:32 [error]: at osu.Framework.Audio.Track.Waveform.<>c__DisplayClass13_0.<.ctor>b__0()
2022-06-26 06:20:32 [error]: at System.Threading.ExecutionContext.RunFromThreadPoolDispatchLoop(Thread threadPoolThread, ExecutionContext executionContext, ContextCallback callback, Object state)
2022-06-26 06:20:32 [error]: --- End of stack trace from previous location ---
2022-06-26 06:20:32 [error]: at System.Threading.Tasks.Task.ExecuteWithThreadLocal(Task& currentTaskSlot, Thread threadPoolThread)
2022-06-26 06:20:32 [error]: --- End of stack trace from previous location ---
2022-06-26 06:20:32 [error]: at osu.Framework.Audio.Track.Waveform.GenerateResampledAsync(Int32 pointCount, CancellationToken cancellationToken)
2022-06-26 06:20:32 [error]: An unobserved error has occurred.
```

smoogipooo/osu-framework,ppy/osu-framework,peppy/osu-framework,ppy/osu-framework,smoogipooo/osu-framework,ppy/osu-framework,peppy/osu-framework,peppy/osu-framework