telnet: Fix telnet segfault when invalid set or help help commands

Silently ignore invalid set ' ' and invalid help help commands.
This is the same fix applied by NetBSD in hg commit 1019940:4f248823eaff.

PR: 265097
Reported by: Simon Josefsson <simon@josefsson.org>
Obtained from: NetBSD hg commit 1019940:4f248823eaff

		NetBSD PR/56918

MFC after: 1 week