[main] Don't watch for keyboard input if no keyboard

In the event should_ignore_show_splash_calls () returns
true, we won't ever have a keyboard set.  This commit
make sure that we don't try to use the keyboard if its
unavailable.

src/main.c

@@ -1510,7 +1510,8 @@ start_boot_splash (state_t    *state,
       return NULL;
     }
 
-  ply_keyboard_watch_for_input (state->keyboard);
+  if (state->keyboard != NULL)
+    ply_keyboard_watch_for_input (state->keyboard);
 
   update_display (state);
   return splash;