Fixed ScriptGroup finalizer for old API

Bug: 28242626

ScriptGroup finalizer clears out the list of closures, which is
uninitialized for the old API.

Need to check null first, before accessing the list.

Change-Id: Ibf914e17a0878c8c561f823c5f6f6f6619594de1

rs/java/android/renderscript/ScriptGroup.java

@@ -1074,7 +1074,11 @@ public final class ScriptGroup extends BaseObj {
     protected void finalize() throws Throwable {
         // Clear out the list mClosures to avoid double-destroying the closures,
         // in case their finalizers race ahead.
-        mClosures.clear();
+        if (mClosures != null) {
+            // ScriptGroup created using the old Builder class does not
+            // initialize the field mClosures
+            mClosures.clear();
+        }
         super.finalize();
     }
 }