Add null check to TaskPersister file list.

Ran out of file descriptors and got a null file list. This
checks for null file list.

Fixes bug 17461213.

Change-Id: Ia6d49d0bdb6da98623b76d2b87cd09bc0afa1f27
diff --git a/services/core/java/com/android/server/am/TaskPersister.java b/services/core/java/com/android/server/am/TaskPersister.java
index 3cc406b..b21af48 100644
--- a/services/core/java/com/android/server/am/TaskPersister.java
+++ b/services/core/java/com/android/server/am/TaskPersister.java
@@ -347,6 +347,10 @@
     private static void removeObsoleteFiles(ArraySet<Integer> persistentTaskIds, File[] files) {
         if (DEBUG) Slog.d(TAG, "removeObsoleteFile: persistentTaskIds=" + persistentTaskIds +
                 " files=" + files);
+        if (files == null) {
+            Slog.e(TAG, "File error accessing recents directory (too many files open?).");
+            return;
+        }
         for (int fileNdx = 0; fileNdx < files.length; ++fileNdx) {
             File file = files[fileNdx];
             String filename = file.getName();