Clean JNI data pointer in shutdown()

Before, finalize() after shutdown() could segfault
since shutdown() kept the pointer to the deleted
SynthProxyJniStorage.

Change-Id: I338c19758733425be2e3b0d9e66176d11d27e8f6
diff --git a/pico/compat/src/com/android/tts/compat/SynthProxy.java b/pico/compat/src/com/android/tts/compat/SynthProxy.java
index 68d0d92..58b2a50 100755
--- a/pico/compat/src/com/android/tts/compat/SynthProxy.java
+++ b/pico/compat/src/com/android/tts/compat/SynthProxy.java
@@ -139,8 +139,9 @@
     /**
      * Shuts down the native synthesizer.
      */
-    public void shutdown()  {
+    public void shutdown() {
         native_shutdown(mJniData);
+        mJniData = 0;
     }
 
     @Override