patch 9.0.0322: crash when no errors and 'quickfixtextfunc' is set

Problem:    Crash when no errors and 'quickfixtextfunc' is set.
Solution:   Do not handle errors if there aren't any.
diff --git a/src/quickfix.c b/src/quickfix.c
index 78f6880..ab288e0 100644
--- a/src/quickfix.c
+++ b/src/quickfix.c
@@ -4761,7 +4761,7 @@
     }
 
     // Check if there is anything to display
-    if (qfl != NULL)
+    if (qfl != NULL && qfl->qf_start != NULL)
     {
 	char_u		dirname[MAXPATHL];
 	int		invalid_val = FALSE;