jni: avoid crashing when run on ART

The ART runtime is stricter about calling env->DeleteWeakGlobalRef
on an object/value that was not returned from env->NewWeakGlobalRef.
In that case, it triggers an abort which causes the application to
crash and display an "Application has stopped" dialog message.

android/jni/jni/eSpeakService.cpp

   if (nat) {
     env->DeleteWeakGlobalRef(nat->object);
     delete nat;
-  } else {
-    env->DeleteWeakGlobalRef(object);
   }
 
   return JNI_TRUE;