When parsing a multiline controller command, be careful with linebreaks

The first line break in particular was mishandled: it was discarded
if no arguments came before it, which made it impossible to
distinguish arguments from the first line of the body.

To solve this, we need to allocate a copy of the command rather than
using NUL to separate it, since we might have "COMMAND\n" as our input.

Fixes ticket 29984.

src/core/mainloop/connection.c

@@ -697,6 +697,7 @@ connection_free_minimal(connection_t *conn)
     control_connection_t *control_conn = TO_CONTROL_CONN(conn);
     tor_free(control_conn->safecookie_client_hash);
     tor_free(control_conn->incoming_cmd);
+    tor_free(control_conn->current_cmd);
     if (control_conn->ephemeral_onion_services) {
       SMARTLIST_FOREACH(control_conn->ephemeral_onion_services, char *, cp, {
         memwipe(cp, 0, strlen(cp));