7145508: java.awt.GraphicsDevice.get/setDisplayMode behavior is incorrect when...

7145508: java.awt.GraphicsDevice.get/setDisplayMode behavior is incorrect when no display is present
Reviewed-by: bae, serb

src/solaris/classes/sun/awt/X11GraphicsDevice.java

@@ -375,7 +375,11 @@ public class X11GraphicsDevice
     @Override
     public synchronized DisplayMode getDisplayMode() {
         if (isFullScreenSupported()) {
-            return getCurrentDisplayMode(screen);
+            DisplayMode mode = getCurrentDisplayMode(screen);
+            if (mode == null) {
+                mode = getDefaultDisplayMode();
+            }
+            return mode;
         } else {
             if (origDisplayMode == null) {
                 origDisplayMode = getDefaultDisplayMode();