Fix build; JDWP::ObjectId feels like a pointer but is actually an int.

Change-Id: I23460f0540115f5cc875f2da178bb99660de1262
diff --git a/src/jdwp/jdwp_handler.cc b/src/jdwp/jdwp_handler.cc
index 7a796fe..36fbaf1 100644
--- a/src/jdwp/jdwp_handler.cc
+++ b/src/jdwp/jdwp_handler.cc
@@ -209,7 +209,7 @@
  */
 static JdwpError VM_AllThreads(JdwpState*, const uint8_t*, int, ExpandBuf* pReply) {
   std::vector<ObjectId> thread_ids;
-  Dbg::GetThreads(NULL, thread_ids);
+  Dbg::GetThreads(0, thread_ids);
 
   expandBufAdd4BE(pReply, thread_ids.size());
   for (uint32_t i = 0; i < thread_ids.size(); ++i) {