Plug a few minor memory leaks, based on a patch by Sirp Potijk.

While I'm here, add an assertion in sshrand.c to catch mistakes in reference
counting.

[originally from svn r8846]

ssh.c

@@ -2853,6 +2853,8 @@ static int ssh_do_close(Ssh ssh, int notify_exit)
 	    del234(ssh->portfwds, pf); /* moving next one to index 0 */
 	    free_portfwd(pf);
 	}
+	freetree234(ssh->portfwds);
+	ssh->portfwds = NULL;
     }
 
     return ret;