FP2-919: "Unfortunately,Gallery has stopped"pops up in the Gallery

when execute resetHistory(),sometimes mMasterImage will be null.if mMasterImage is null,may lead to java.lang.NullPointerException

Change-Id: I7cda813090f2d9670edafb0624b439e52ec1c98f
diff --git a/src/com/android/gallery3d/filtershow/FilterShowActivity.java b/src/com/android/gallery3d/filtershow/FilterShowActivity.java
index 7bef513..bc6c51c 100644
--- a/src/com/android/gallery3d/filtershow/FilterShowActivity.java
+++ b/src/com/android/gallery3d/filtershow/FilterShowActivity.java
@@ -1295,6 +1295,9 @@
     }
 
     void resetHistory() {
+        if (mMasterImage == null) {
+            return;
+        }
         HistoryManager adapter = mMasterImage.getHistory();
         adapter.reset();
         HistoryItem historyItem = adapter.getItem(0);