patch 9.0.1857: [security] heap-use-after-free in is_qf_win()

Problem:  heap-use-after-free in is_qf_win()
Solution: Check buffer is valid before accessing it

Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/main.c b/src/main.c
index cca53fe..a40c3a6 100644
--- a/src/main.c
+++ b/src/main.c
@@ -1646,7 +1646,7 @@
 	    next_tp = tp->tp_next;
 	    FOR_ALL_WINDOWS_IN_TAB(tp, wp)
 	    {
-		if (wp->w_buffer == NULL)
+		if (wp->w_buffer == NULL || !buf_valid(wp->w_buffer))
 		    // Autocmd must have close the buffer already, skip.
 		    continue;
 		buf = wp->w_buffer;