Don't try to take log2 of 0 when using mono audio channels.

[jdm]

This avoids a panic when loading rooms in Hubs.

[highfive]

Heads up! This PR modifies the following files:

• @asajeffrey: components/script/dom/baseaudiocontext.rs
• @KiChjang: components/script/dom/baseaudiocontext.rs

[jdm]

@bors-servo try=wpt

[highfive]

Warning

• These commits modify script code, but no tests are modified. Please consider adding a test!

[bors-servo]

⌛ Trying commit b643451 with merge 5f25b15...

[bors-servo]

☀️ Test successful - status-taskcluster
State: approved= try=True

[Manishearth]

We also shouldn't be using log2 at all, ideally we can do (x - 1).count_ones() + 1, conditional on x.count_ones() being one

[jdm]

I now can't reproduce this panic on Hubs without this PR applied, and I haven't been able to reproduce the panic in a page using a mono audio buffer:

<script>
var context = new AudioContext();
var buffer = context.createBuffer(1, 22050, 22050);
for (var channel = 0; channel < buffer.numberOfChannels; channel++) {
  // This gives us the actual ArrayBuffer that contains the data
  var nowBuffering = buffer.getChannelData(channel);
  for (var i = 0; i < buffer.length; i++) {
    // Math.random() is in [0; 1.0]
    // audio needs to be in [-1.0; 1.0]
    nowBuffering[i] = Math.random() * 2 - 1;
  }
}
var source = context.createBufferSource();
source.buffer = buffer;
source.connect(context.destination);
source.start();
</script>

Since I don't really want to merge changes that can't clearly show an improvement, I'm going to close this PR. We can reopen it if we find a way to trigger the problem again.