MINOR: server: check if srv is NULL in free_server()

Check if srv is NULL before trying to do anything  in free_server(),
like most free()-like function do.
diff --git a/src/server.c b/src/server.c
index f1e294a..7cec4ce 100644
--- a/src/server.c
+++ b/src/server.c
@@ -2216,6 +2216,9 @@
  */
 void free_server(struct server *srv)
 {
+	if (!srv)
+		return;
+
 	/* For dynamic servers, decrement the reference counter. Only free the
 	 * server when reaching zero.
 	 */