Fix a hang on servers that don't support the X Keyboard Extension

* src/xfns.c (Fx_backspace_delete_keys_p): Don't block input
before checking for Xkb.

src/xfns.c

@@ -7749,11 +7749,11 @@ present and mapped to the usual X keysyms.  */)
   Display *dpy = FRAME_X_DISPLAY (f);
   Lisp_Object have_keys;
 
-  block_input ();
-
   if (!FRAME_DISPLAY_INFO (f)->supports_xkb)
     return Qlambda;
 
+  block_input ();
+
   /* In this code we check that the keyboard has physical keys with names
      that start with BKSP (Backspace) and DELE (Delete), and that they
      generate keysym XK_BackSpace and XK_Delete respectively.