Android: Fix synchronization problems in VideoFrame release.

webrtc/sdk/android/src/java/org/webrtc/NV12Buffer.java

Index: webrtc/sdk/android/src/java/org/webrtc/NV12Buffer.java
diff --git a/webrtc/sdk/android/src/java/org/webrtc/NV12Buffer.java b/webrtc/sdk/android/src/java/org/webrtc/NV12Buffer.java
index 7575929cba7bf70483b6b47c3d01a369e086ece4..af9ac253f17e40497004427706b72044647d7634 100644
--- a/webrtc/sdk/android/src/java/org/webrtc/NV12Buffer.java
+++ b/webrtc/sdk/android/src/java/org/webrtc/NV12Buffer.java
@@ -19,6 +19,7 @@ public class NV12Buffer implements VideoFrame.Buffer {
   private final int sliceHeight;
   private final ByteBuffer buffer;
   private final Runnable releaseCallback;
+  private final Object refCountLock = new Object();
 
   private int refCount;
 
@@ -51,13 +52,17 @@ public class NV12Buffer implements VideoFrame.Buffer {
 
   @Override
   public void retain() {
-    refCount++;
+    synchronized (refCountLock) {
+      ++refCount;
+    }
   }
 
   @Override
   public void release() {
-    if (--refCount == 0) {
-      releaseCallback.run();
+    synchronized (refCountLock) {
+      if (--refCount == 0 && releaseCallback != null) {
+        releaseCallback.run();
+      }
     }
   }
 
@@ -75,4 +80,4 @@ public class NV12Buffer implements VideoFrame.Buffer {
       int scaleWidth, int scaleHeight, ByteBuffer src, int srcWidth, int srcHeight, int srcStride,
       int srcSliceHeight, ByteBuffer dstY, int dstStrideY, ByteBuffer dstU, int dstStrideU,
       ByteBuffer dstV, int dstStrideV);
-}
+}