patch 8.1.1015: quickfix buffer shows up in list, can't get buffer number

patch 8.1.1015: quickfix buffer shows up in list, can't get buffer number

Problem:    Quickfix buffer shows up in list, can't get buffer number.
Solution:   Make the quickfix buffer unlisted when the quickfix window is
            closed.  get the quickfix buffer number with getqflist().
            (Yegappan Lakshmanan, closes #4113)