patch 8.1.1549: quickfix test fails

Problem:    Quickfix test fails.
Solution:   Negate result of bt_quickfix().
diff --git a/src/quickfix.c b/src/quickfix.c
index d2d3302..8c25915 100644
--- a/src/quickfix.c
+++ b/src/quickfix.c
@@ -4135,7 +4135,7 @@
     // Set the options for the quickfix buffer/window (if not already done)
     // Do this even if the quickfix buffer was already present, as an autocmd
     // might have previously deleted (:bdelete) the quickfix buffer.
-    if (bt_quickfix(curbuf))
+    if (!bt_quickfix(curbuf))
 	qf_set_cwindow_options();
 
     // Only set the height when still in the same tab page and there is no