Set nativeObserver to 0 to avoid double release.

Bug: webrtc:12769
Change-Id: Ifcd3e8148e999740a697b27ddd12f6aca36c5440
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/218780
Commit-Queue: Tian Tan <tiantan@google.com>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#34029}

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

@@ -123,6 +123,7 @@ public class DataChannel {
   public void unregisterObserver() {
     checkDataChannelExists();
     nativeUnregisterObserver(nativeObserver);
+    nativeObserver = 0;
   }
 
   public String label() {