Fix bug preventing master volume from being set to 0

Author: goldfire

howler.js

@@ -58,7 +58,7 @@
       // make sure volume is a number
       vol = parseFloat(vol);
 
-      if (vol && vol >= 0 && vol <= 1) {
+      if (vol >= 0 && vol <= 1) {
         self._volume = vol;
 
         if (usingWebAudio) {