patch 8.1.1134: buffer for quickfix window is reused for another file

Problem:    Buffer for quickfix window is reused for another file.
Solution:   Don't reuse the quickfx buffer. (Yegappan Lakshmanan)

src/testdir/test_quickfix.vim

@@ -4012,3 +4012,17 @@ func Test_winonly_autocmd()
   new | only
   call delete('Xtest1')
 endfunc
+
+" Test to make sure that an empty quickfix buffer is not reused for loading
+" a normal buffer.
+func Test_empty_qfbuf()
+  enew | only
+  call writefile(["Test"], 'Xfile1')
+  call setqflist([], 'f')
+  copen | only
+  let qfbuf = bufnr('')
+  edit Xfile1
+  call assert_notequal(qfbuf, bufnr(''))
+  enew
+  call delete('Xfile1')
+endfunc