Espresso: JDWP: Check if the owner thread is null before trying to convert it.

Author: mikehearn

espresso/src/com.oracle.truffle.espresso/src/com/oracle/truffle/espresso/runtime/JDWPContextImpl.java

@@ -666,7 +666,10 @@ public CallFrame locateObjectWaitFrame() {
     public Object getMonitorOwnerThread(Object object) {
         if (object instanceof StaticObject) {
             EspressoLock lock = ((StaticObject) object).getLock(context);
-            return asGuestThread(lock.getOwnerThread());
+            Thread ownerThread = lock.getOwnerThread();
+            if (ownerThread != null) {
+                return asGuestThread(ownerThread);
+            }
         }
         return null;
     }