MINOR: connection: make the debugging helper functions safer

We have various functions like conn_get_ctrl_name() to retrieve
some information reported in "show sess" for debugging, which
assume that the connection is valid. This is really not convenient
in code aimed at debugging and is error-prone. Let's add a validity
test first.
diff --git a/include/proto/connection.h b/include/proto/connection.h
index 0db7c47..f1919ce 100644
--- a/include/proto/connection.h
+++ b/include/proto/connection.h
@@ -894,28 +894,28 @@
 
 static inline const char *conn_get_ctrl_name(const struct connection *conn)
 {
-	if (!conn_ctrl_ready(conn))
+	if (!conn || !conn_ctrl_ready(conn))
 		return "NONE";
 	return conn->ctrl->name;
 }
 
 static inline const char *conn_get_xprt_name(const struct connection *conn)
 {
-	if (!conn_xprt_ready(conn))
+	if (!conn || !conn_xprt_ready(conn))
 		return "NONE";
 	return conn->xprt->name;
 }
 
 static inline const char *conn_get_mux_name(const struct connection *conn)
 {
-	if (!conn->mux)
+	if (!conn || !conn->mux)
 		return "NONE";
 	return conn->mux->name;
 }
 
 static inline const char *cs_get_data_name(const struct conn_stream *cs)
 {
-	if (!cs->data_cb)
+	if (!cs || !cs->data_cb)
 		return "NONE";
 	return cs->data_cb->name;
 }