lines.between() does not check if the end marker actually exists, but
later it is assumed the marker exists. This change introduces a check
for the end marker and doesn't decrypt messages in which the end marker
is missing. This is a fix for
http://rubyforge.org/pipermail/sup-talk/2010-August/00420
Excerpts from Michael Hamann's message of Sat Sep 04 17:51:46 -0400 2010:
> lines.between() does not check if the end marker actually exists, but
> later it is assumed the marker exists. This change introduces a check
> for the end marker and doesn't decrypt messages in which the end marker
> is mi
