Small bugfixes to MakeListDict in orgcheck.vim

- "exec lineno + 1" didn't move the cursor, causing an infinite loop,
  changed to setpos.
- if the function is called from a non-list line, this_indent did not
  exist yet, making "let last_indent = this_indent" give an error.
- if there is no line with only whitespace after the starting position,
  then the loop does not finish.