If the number of messages in a folder is zero, then don't try to

If the number of messages in a folder is zero, then don't try to
clear out the sequence list; this can cause you to run off the
beginning of the malloc'd buffer.  This can happen when you use
"comp" with a draft folder, the draft folder is empty, and you have
a previous sequence set in your .mh_profile.  As far as I can tell
this has been a problem for nearly forever, but I only ran into it
recently when adding the readline support.  Go figure.