Xi: handle allocation failure in add_master_func()

Reported in #1817:

xwayland-24.1.6/redhat-linux-build/../Xi/xibarriers.c:729:5:
 warning[-Wanalyzer-null-dereference]:
 dereference of NULL ‘AllocBarrierDevice()’

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Part-of: <https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/2075>
(cherry picked from commit 0ee6039053)

Xi/xibarriers.c

@@ -726,6 +726,8 @@ static void add_master_func(void *res, XID id, void *devid)
 
 
     pbd = AllocBarrierDevice();
+    if (!pbd)
+        return;
     pbd->deviceid = *deviceid;
 
     input_lock();