Correctly handle notify before wait for virtual threads

If a notify is issued before wait, then there waitingContinuations
will be NULL. In this case, the notify steps should not be executed
since there are no virtual threads waiting, and
waitingContinuations->vthread will be NULL.

Addresses point 3 in the below Github comment:
eclipse-openj9#20369 (comment).

Signed-off-by: Babneet Singh <[email protected]>

Author: babsingh

runtime/vm/BytecodeInterpreter.hpp

@@ -2943,7 +2943,7 @@ class INTERPRETER_CLASS
 					if ((NULL != objectMonitor) && (NULL != objectMonitor->waitingContinuations)) {
 						omrthread_monitor_enter(_vm->blockedVirtualThreadsMutex);
 						J9VMContinuation *head = objectMonitor->waitingContinuations;
-						if (NULL != head) {
+						if ((NULL != head) && (NULL != head->vthread)) {
 							if (omrthread_monitor_notify == notifyFunction) {
 								objectMonitor->waitingContinuations = head->nextWaitingContinuation;
 								head->nextWaitingContinuation = _vm->blockedContinuations;