Fix race condition with preview callback.

Bug: 6301018
Change-Id: I19b8bc03ef3983d69e1e986c0f1a8154af5ab080

Author: Eino-Ville Talvala

core/java/android/hardware/Camera.java

@@ -736,8 +736,8 @@ public void handleMessage(Message msg) {
                 return;
 
             case CAMERA_MSG_PREVIEW_FRAME:
-                if (mPreviewCallback != null) {
-                    PreviewCallback cb = mPreviewCallback;
+                PreviewCallback pCb = mPreviewCallback;
+                if (pCb != null) {
                     if (mOneShot) {
                         // Clear the callback variable before the callback
                         // in case the app calls setPreviewCallback from
@@ -749,7 +749,7 @@ public void handleMessage(Message msg) {
                         // Set to oneshot mode again.
                         setHasPreviewCallback(true, false);
                     }
-                    cb.onPreviewFrame((byte[])msg.obj, mCamera);
+                    pCb.onPreviewFrame((byte[])msg.obj, mCamera);
                 }
                 return;