Check position against null, not zero

a8ac314 had a bug in which a switch at position zero would not result
in clearing anything.  Positions may validly be zero, but never null.

Change-Id: I95a607df7bd6a41cf73f78122c3dcdda425fb145

lib/media/streaming_engine.js

@@ -478,14 +478,14 @@ shaka.media.StreamingEngine.prototype.clear_ = function(mediaState, clear) {
 
     goog.asserts.assert(position != null,
         'No segment under the playhead! This should not happen!');
-    if (!position) {
+    if (position == null) {
       return;
     }
 
     var reference = mediaState.stream.getSegmentReference(position);
     goog.asserts.assert(reference != null,
         'No reference for current position! This should not happen!');
-    if (!reference) {
+    if (reference == null) {
       return;
     }