Don't try to set the length, end, or text of a faked item.

If we try we may (in the case of setting the length or end) mistakenly assert
out.

Bug: 10329
Change-Id: I52083ba17f6427712a6a363f48f1f7ae51f2b74b
Reviewed-on: https://code.wireshark.org/review/4093
Reviewed-by: Evan Huus <eapache@gmail.com>
Reviewed-by: Jeff Morriss <jeff.morriss.ws@gmail.com>