vim-patch:8.2.1905: the wininfo list may contain stale entries (#14884)

Problem:    The wininfo list may contain stale entries.
Solution:   When closing a window remove any other entry where the window
            pointer is NULL.
4882d98339

test/functional/legacy/memory_usage_spec.lua

@@ -166,4 +166,36 @@ describe('memory usage', function()
     check_result({before=before, after=after, last=last},
                  pcall(ok, last.last < upper))
   end)
+
+  it('releases memory when closing windows when folds exist', function()
+    local pid = eval('getpid()')
+    source([[
+      new
+      " Insert lines
+      call nvim_buf_set_lines(0, 0, 0, v:false, repeat([''], 999))
+      " Create folds
+      normal! gg
+      for _ in range(500)
+        normal! zfjj
+      endfor
+    ]])
+    poke_eventloop()
+    local before = monitor_memory_usage(pid)
+    source([[
+      " Split and close window multiple times
+      for _ in range(1000)
+        split
+        close
+      endfor
+    ]])
+    poke_eventloop()
+    local after = monitor_memory_usage(pid)
+    source('bwipe!')
+    poke_eventloop()
+    -- Allow for an increase of 5% in memory usage, which accommodates minor fluctuation,
+    -- but is small enough that if memory were not released (prior to PR #14884), the test
+    -- would fail.
+    local upper = before.last * 1.05
+    check_result({before=before, after=after}, pcall(ok, after.last <= upper))
+  end)
 end)