Xext/xselinux: add fast path to ProcSELinuxListSelections()

If there's nothing to send, skip over a bunch of code to make a list
that won't be used, and hopefully make the code path clearer to both
humans and static analyzers, who raise errors as seen in #1817 of
dereferencing NULL pointers when count == 0.

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Part-of: <https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/2072>
(cherry picked from commit d34243606c)

Xext/xselinux_ext.c

@@ -455,8 +455,10 @@ ProcSELinuxListSelections(ClientPtr client)
     count = 0;
     for (pSel = CurrentSelections; pSel; pSel = pSel->next)
         count++;
+    if (count == 0)
+        return SELinuxSendItemsToClient(client, NULL, 0, 0);
     items = calloc(count, sizeof(SELinuxListItemRec));
-    if (count && !items)
+    if (!items)
         return BadAlloc;
 
     /* Fill in the items and calculate size */