Throw exception if MediaStreamTrack is constructed with a null native track.

Bug: webrtc:7543, webrtc:7566
Change-Id: I71f3ba1d6d77e51a09b0659e35eb30845b9fca91
Reviewed-on: https://webrtc-review.googlesource.com/102410
Commit-Queue: Steve Anton <steveanton@webrtc.org>
Reviewed-by: Sami Kalliomäki <sakal@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24898}

sdk/android/api/org/webrtc/MediaStreamTrack.java

@@ -73,6 +73,9 @@ public class MediaStreamTrack {
   private long nativeTrack;
 
   public MediaStreamTrack(long nativeTrack) {
+    if (nativeTrack == 0) {
+      throw new IllegalArgumentException("nativeTrack may not be null");
+    }
     this.nativeTrack = nativeTrack;
   }