Fix error when seeking with unloaded state

[Stenerson]

This is a fix for #796

We could also check the length of ids but I think the intention is more clear by using self._sounds.

If the state is unloaded there will be no _sounds so the id will be left as undefined and the function will bail on line 1370:

1368:      // If there is no ID, bail out.
1369:      if (typeof id === 'undefined') {
1370:        return self;
1371:      }

[goldfire]

Thanks!