dix: clear free()ed pointers on CloseDownExtensions()

As safety precaution, clear the pointers to extion records that just
have been free()ed.

Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>

dix/extension.c

@@ -203,6 +203,7 @@ CloseDownExtensions(void)
         free((void *) extensions[i]->name);
         dixFreePrivates(extensions[i]->devPrivates, PRIVATE_EXTENSION);
         free(extensions[i]);
+        extensions[i] = NULL;
     }
     free(extensions);
     extensions = (ExtensionEntry **) NULL;