Fix leaks when audio context cannot be unlocked (MacOS Safari)

Fix creating unbounded number of Audio elements when `unlock` is called multiple times
Fix calling createBufferSource() on every touch event if not unlocked

src/howler.core.js

@@ -326,7 +326,7 @@
         // to the WebAudio API which only needs a single activation.
         // This must occur before WebAudio setup or the source.onended
         // event will not fire.
-        for (var i=0; i<self.html5PoolSize; i++) {
+        while (self._html5AudioPool.length < self.html5PoolSize) {
           try {
             var audioNode = new Audio();
 
@@ -338,6 +338,7 @@
             self._releaseHtml5Audio(audioNode);
           } catch (e) {
             self.noAudio = true;
+            break;
           }
         }
 
@@ -363,9 +364,12 @@
         self._autoResume();
 
         // Create an empty buffer.
-        var source = self.ctx.createBufferSource();
-        source.buffer = self._scratchBuffer;
-        source.connect(self.ctx.destination);
+        var source = self._unlockSourceBuffer;
+        if (!source) {
+          source = self._unlockSourceBuffer = self.ctx.createBufferSource();
+          source.buffer = self._scratchBuffer;
+          source.connect(self.ctx.destination);
+        }
 
         // Play the empty buffer.
         if (typeof source.start === 'undefined') {
@@ -381,6 +385,7 @@
 
         // Setup a timeout to check that we are unlocked on the next event loop.
         source.onended = function() {
+          delete self._unlockSourceBuffer;
           source.disconnect(0);
 
           // Update the unlocked state and prevent this check from happening again.