patch 8.2.1905: the wininfo list may contain stale entries

Problem:    The wininfo list may contain stale entries.
Solution:   When closing a window remove any other entry where the window
            pointer is NULL.
diff --git a/src/proto/buffer.pro b/src/proto/buffer.pro
index 3a63299..4d890ce 100644
--- a/src/proto/buffer.pro
+++ b/src/proto/buffer.pro
@@ -8,6 +8,7 @@
 void close_buffer(win_T *win, buf_T *buf, int action, int abort_if_last, int ignore_abort);
 void buf_clear_file(buf_T *buf);
 void buf_freeall(buf_T *buf, int flags);
+void free_wininfo(wininfo_T *wip);
 void goto_buffer(exarg_T *eap, int start, int dir, int count);
 void handle_swap_exists(bufref_T *old_curbuf);
 char *do_bufdel(int command, char_u *arg, int addr_count, int start_bnr, int end_bnr, int forceit);