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)
diff --git a/src/window.c b/src/window.c
index 98d6085..6c9c705 100644
--- a/src/window.c
+++ b/src/window.c
@@ -2382,18 +2382,24 @@
     }
 
 #ifdef FEAT_GUI
-    /* Avoid trouble with scrollbars that are going to be deleted in
-     * win_free(). */
+    // Avoid trouble with scrollbars that are going to be deleted in
+    // win_free().
     if (gui.in_use)
 	out_flush();
 #endif
 
 #ifdef FEAT_SYN_HL
-    /* Free independent synblock before the buffer is freed. */
+    // Free independent synblock before the buffer is freed.
     if (win->w_buffer != NULL)
 	reset_synblock(win);
 #endif
 
+#ifdef FEAT_QUICKFIX
+    // When the quickfix/location list window is closed, unlist the buffer.
+    if (win->w_buffer != NULL && bt_quickfix(win->w_buffer))
+	win->w_buffer->b_p_bl = FALSE;
+#endif
+
     /*
      * Close the link to the buffer.
      */