Cleanup JDWP event matching

* Use std::vector for the event match list.
* Make event reporting methods void since result is never used.
* Use nullptr keyword instead of NULL.

Change-Id: Icd6f47e46cefc2cc63325df00037cd4b6a475259
diff --git a/runtime/jdwp/object_registry.cc b/runtime/jdwp/object_registry.cc
index 9123994..20db368 100644
--- a/runtime/jdwp/object_registry.cc
+++ b/runtime/jdwp/object_registry.cc
@@ -138,7 +138,7 @@
 
 jobject ObjectRegistry::GetJObject(JDWP::ObjectId id) {
   if (id == 0) {
-    return NULL;
+    return nullptr;
   }
   Thread* self = Thread::Current();
   MutexLock mu(self, lock_);
@@ -194,7 +194,7 @@
   ObjectRegistryEntry& entry = *it->second;
   if (entry.jni_reference_type == JNIWeakGlobalRefType) {
     JNIEnv* env = self->GetJniEnv();
-    return env->IsSameObject(entry.jni_reference, NULL);  // Has the jweak been collected?
+    return env->IsSameObject(entry.jni_reference, nullptr);  // Has the jweak been collected?
   } else {
     return false;  // We hold a strong reference, so we know this is live.
   }