WallpaperManager: add NULL pointer check for getCropAndSetWallpaperIntent

Originally the api 'getCropAndSetWallpaperIntent' does not check
whether the parsed parameter is a NULL pointer, as leads to the
whole app crash if it call this api with a null parameter accidentally.
A null pointer check logic is added into its original parameter check
logic, so that this API will throw an IllegalArgumentException if it
is parsed with a null pointer

Change-Id: Ib7de40e571419d09e1a744edc969eb7162766b75

core/java/android/app/WallpaperManager.java

@@ -668,6 +668,10 @@ public class WallpaperManager {
      *         not "image/*"
      */
     public Intent getCropAndSetWallpaperIntent(Uri imageUri) {
+        if (imageUri == null) {
+            throw new IllegalArgumentException("Image URI must not be null");
+        }
+
         if (!ContentResolver.SCHEME_CONTENT.equals(imageUri.getScheme())) {
             throw new IllegalArgumentException("Image URI must be of the "
                     + ContentResolver.SCHEME_CONTENT + " scheme type");