Fix jvmtiGetVirtualThread and make it more robust

If no virtual thread is mounted on the targetThread i.e.
carrierThreadObject == threadObject, then return null
for the output virtual thread value.

Virtual thread is not pinned in this function. So, read
threadObject and carrierThreadObject once and store their
values to avoid inconsistency.

Related: eclipse-openj9#16688
Related: eclipse-openj9#16751

Signed-off-by: Babneet Singh <sbabneet@ca.ibm.com>

runtime/jvmti/jvmtiExtensionMechanism.c

@@ -3805,10 +3805,14 @@ jvmtiGetVirtualThread(jvmtiEnv* jvmti_env, ...)
 				currentThread, carrier_thread, &targetThread, JVMTI_ERROR_NONE,
 				J9JVMTI_GETVMTHREAD_ERROR_ON_DEAD_THREAD);
 		if (JVMTI_ERROR_NONE == rc) {
-			if (NULL != targetThread->threadObject) {
+			j9object_t threadObject = targetThread->threadObject;
+			j9object_t carrierThreadObject = targetThread->carrierThreadObject;
+			if ((NULL != threadObject)
+			&& (threadObject != carrierThreadObject)
+			) {
 				rv_virtual_thread = (jthread)vm->internalVMFunctions->j9jni_createLocalRef(
 												(JNIEnv *)currentThread,
-												(j9object_t)targetThread->threadObject);
+												(j9object_t)threadObject);
 			}
 			releaseVMThread(currentThread, targetThread, carrier_thread);
 		}