Unloading an unloaded howl causes a different howl to play silently

[mtsanford]

Steps:

1. 2 howls with preload = false, same src
2. howl1.load()
3. after a delay, howl2.unload()
4. howl1.play()

howl2 plays, but it is silent

https://codepen.io/marktsanford/pen/GPrpgv

Note: this did not happen in 2.0.12, but does in all subsequent releases.

[mtsanford]

I see what's going on. Line 1728.

      // Delete this sound from the cache (if no other Howl is using it).
      var remCache = true;
      for (i=0; i<Howler._howls.length; i++) {
        if (Howler._howls[i]._src === self._src) {
          remCache = false;
          break;
        }
      }

      if (cache && remCache) {
        delete cache[self._src];
      }

The unload function does not check to see if the howl is loaded. If it's not, the _src property is still an array and this check will fail:

if (Howler._howls[i]._src === self._src)

And this will delete from the cache.

// counterintuitively, cache['somestring'] === cache[['somestring']]
// so deleting with the _src array as index works when the array only has one string
delete cache[self._src];

Howl unload function needs to handle the case where the howl has never been loaded, to avoid erroneous removal from the cache.

I can make pull request if you like, but I'm not sure what the best way to handle this is.

[mtsanford]

Workaround is to make sure the howl is not unloaded before unloading it.

if (howl.state() != 'unloaded') howl.unload()