patch 9.1.0149: null pointer member access when accessing 'winfixbuf' property

Problem:  qf_goto_win_with_qfl_file may check if prevwin has 'winfixbuf'
          set without checking if it's valid first.
Solution: Reverse the condition. Add a test, a modeline, and a missing
          CheckFeature. (Searn Dewar)

closes: #14140

Signed-off-by: Sean Dewar <6256228+seandewar@users.noreply.github.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/quickfix.c b/src/quickfix.c
index 1f4176f..02a69d2 100644
--- a/src/quickfix.c
+++ b/src/quickfix.c
@@ -3146,7 +3146,7 @@
 	    // Didn't find it, go to the window before the quickfix
 	    // window, unless 'switchbuf' contains 'uselast': in this case we
 	    // try to jump to the previously used window first.
-	    if ((swb_flags & SWB_USELAST) && !prevwin->w_p_wfb && win_valid(prevwin))
+	    if ((swb_flags & SWB_USELAST) && win_valid(prevwin) && !prevwin->w_p_wfb)
 		win = prevwin;
 	    else if (altwin != NULL)
 		win = altwin;