Don't crash if the stack state is empty.

This only happens when throwables override fillInStackTrace().

Change-Id: I4513b53491e293725ba9e914228bdc19526ab874
diff --git a/src/java_lang_Throwable.cc b/src/java_lang_Throwable.cc
index e8a3891..2ab4ed6 100644
--- a/src/java_lang_Throwable.cc
+++ b/src/java_lang_Throwable.cc
@@ -29,6 +29,9 @@
 }
 
 jobjectArray Throwable_nativeGetStackTrace(JNIEnv* env, jclass, jobject javaStackState) {
+  if (javaStackState == NULL) {
+      return NULL;
+  }
   return Thread::InternalStackTraceToStackTraceElementArray(env, javaStackState);
 }