xace: ricer tuning for XaceHook

gcc doesn't want to hoist the check for XaceHooks[hook] != NULL above the
varargs code for some reason, so do it ourselves.

Before:
40000000 trep @   0.0010 msec (1050420.2/sec): PutImage 10x10 square
60000000 trep @   0.0005 msec (1921147.6/sec): ShmPutImage 10x10 square

After:
40000000 trep @   0.0009 msec (1087458.5/sec): PutImage 10x10 square
60000000 trep @   0.0005 msec (2012238.6/sec): ShmPutImage 10x10 square

Reviewed-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Signed-off-by: Adam Jackson <ajax@redhat.com>
(cherry picked from commit 11977fab54)

Xext/xace.c

@@ -101,6 +101,10 @@ int XaceHook(int hook, ...)
     } u;
     int *prv = NULL;	/* points to return value from callback */
     va_list ap;		/* argument list */
+
+    if (!XaceHooks[hook])
+	return Success;
+
     va_start(ap, hook);
 
     /* Marshal arguments for passing to callback.