check for active_console before using it

Other vga_hw_* functions do the same.
Fixes a segmentation fault.  Trigger: boot with -nodefaults,
then connect via vnc.
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

console.c

@@ -167,7 +167,7 @@ void vga_hw_update(void)
 
 void vga_hw_invalidate(void)
 {
-    if (active_console->hw_invalidate)
+    if (active_console && active_console->hw_invalidate)
         active_console->hw_invalidate(active_console->hw);
 }