[Win32] Add double-dispose guard in DestroyableImageHandle.destroy()

The destroy() method did not check the isDisposed flag before calling
OS.DestroyIcon/OS.DeleteObject. If called twice, it would pass an
already-freed handle to the OS, risking undefined behavior. Add an
early return when the handle is already disposed.

Author: HeikoKlare

bundles/org.eclipse.swt/Eclipse SWT/win32/org/eclipse/swt/graphics/Image.java

@@ -3319,6 +3319,7 @@ public boolean isDisposed() {
 	}
 
 	void destroy() {
+		if (isDisposed) return;
 		if (type == SWT.ICON) {
 			OS.DestroyIcon (handle());
 		} else {