Redefine clients as a fixed array

This removes yet another xalloc() each server generation. Also, I
couldn't find the corresponding xfree() so I guess that used to be a
memory leak there.

include/dix.h

@@ -119,7 +119,7 @@ typedef struct _Client *ClientPtr; /* also in misc.h */
 
 typedef struct _WorkQueue	*WorkQueuePtr;
 
-extern ClientPtr *clients;
+extern ClientPtr clients[MAXCLIENTS];
 extern ClientPtr serverClient;
 extern int currentMaxClients;
 extern char dispatchExceptionAtReset;