Fix a race in indy global var caching

The logic here acquires the Invalidator from the variable and uses
that to guard the cached value. However, if this is a SwitchPoint
invalidator (assumed by the code) then it replaces the SwitchPoint
with a new one after invalidation. If we do not grab the actual
SwitchPoint before the value, we may end up associating the
current value with a newer SwitchPoint and never see the
subsequent update.

This is likely the main cause of intermittent global variable
failures with indy global caching turned on.

core/src/main/java/org/jruby/runtime/invokedynamic/GlobalSite.java

@@ -105,15 +105,15 @@ public IRubyObject getGlobalFallback(ThreadContext context) throws Throwable {
             return (IRubyObject)uncached.invokeWithArguments(context);
         }
 
-        Invalidator invalidator = variable.getInvalidator();
+        SwitchPoint invalidator = (SwitchPoint) variable.getInvalidator().getData();
         IRubyObject value = variable.getAccessor().getValue();
 
         MethodHandle target = constant(IRubyObject.class, value);
         target = dropArguments(target, 0, ThreadContext.class);
         MethodHandle fallback = lookup().findVirtual(GlobalSite.class, "getGlobalFallback", methodType(IRubyObject.class, ThreadContext.class));
         fallback = fallback.bindTo(this);
 
-        target = ((SwitchPoint)invalidator.getData()).guardWithTest(target, fallback);
+        target = invalidator.guardWithTest(target, fallback);
 
         setTarget(target);