xkb: if kbd init failed, NULL out the pointers after freeing them (#21278)

Reproducible:
Configure a server that uses the keyboard driver with an invalid ruleset,
e.g. (Option "XkbRules" "foobar"). Ensure that Option "AllowEmptyInput" is
"off" in the ServerFlags or ServerLayout section. Start the server.
After failing to init the keymap, the server will try to clean up after the
device, double-freeing some xkb structs that have not been reset properly.

X.Org Bug 21278 <http://bugs.freedesktop.org/show_bug.cgi?id=21278>

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>

xkb/xkbInit.c

@@ -622,10 +622,13 @@ unwind_desc:
     XkbFreeKeyboard(xkb, 0, TRUE);
 unwind_info:
     xfree(xkbi);
+    dev->key->xkbInfo = NULL;
 unwind_kbdfeed:
     xfree(dev->kbdfeed);
+    dev->kbdfeed = NULL;
 unwind_key:
     xfree(dev->key);
+    dev->key = NULL;
     return FALSE;
 }