6990352: SplashScreen.getSplashScreen() does not return null for implicitly closed splash screen

Summary: Mark the splash screen closed when it happens implicitly
Reviewed-by: art, dcherepanov

src/share/classes/java/awt/SplashScreen.java

@@ -318,6 +318,12 @@ public final class SplashScreen {
             checkVisible();
             _close(splashPtr);
             image = null;
+            SplashScreen.markClosed();
+        }
+    }
+
+    static void markClosed() {
+        synchronized (SplashScreen.class) {
             wasClosed = true;
             theInstance = null;
         }

src/share/classes/java/awt/Window.java

@@ -928,7 +928,10 @@ public class Window extends Container implements Accessible {
             return;
         }
         if (beforeFirstWindowShown.getAndSet(false)) {
+            // We don't use SplashScreen.getSplashScreen() to avoid instantiating
+            // the object if it hasn't been requested by user code explicitly
             SunToolkit.closeSplashScreen();
+            SplashScreen.markClosed();
         }
     }