session: Fix a memory leak with custom banner.

BUG: https://red.libssh.org/issues/152

src/session.c

@@ -261,6 +261,7 @@ void ssh_free(ssh_session session) {
   SAFE_FREE(session->banner);
 
   SAFE_FREE(session->opts.bindaddr);
+  SAFE_FREE(session->opts.custombanner);
   SAFE_FREE(session->opts.username);
   SAFE_FREE(session->opts.host);
   SAFE_FREE(session->opts.sshdir);