Remove redundant mbox delimiter check in imap_browse(). (closes #3646)

Remove redundant mbox delimiter check in imap_browse().  (closes #3646)

imap_fix_path() removes duplicate and trailing delimiters, so the check
below it was redundant.  This also made it appear list.delim could be
used uninitialized.

Remove the check, but add a check to make sure the "fixed" path has
len>0, to prevent oob accesses of mbox[n-1] below.

Lastly, remove a redundant n=strlen(mbox) inside the initial LIST
processing loop.  The mbox isn't changed from above, so there is no
need to rerun strlen.