MINOR: proxy: check if p is NULL in free_proxy()

Check if p is NULL before trying to do anything  in free_proxy(),
like most free()-like function do.
diff --git a/src/proxy.c b/src/proxy.c
index 237f297..87999ad 100644
--- a/src/proxy.c
+++ b/src/proxy.c
@@ -144,6 +144,9 @@
 	struct proxy_deinit_fct *pxdf;
 	struct server_deinit_fct *srvdf;
 
+	if (!p)
+		return;
+
 	free(p->conf.file);
 	free(p->id);
 	free(p->cookie_name);