patch 8.2.1255: cannot use a lambda with quickfix functions

Problem:    Cannot use a lambda with quickfix functions.
Solution:   Add support for lambda. (Yegappan Lakshmanan, closes #6499)
diff --git a/src/optionstr.c b/src/optionstr.c
index bfc2e3b..177ce85 100644
--- a/src/optionstr.c
+++ b/src/optionstr.c
@@ -2255,6 +2255,14 @@
 # endif
 #endif
 
+#ifdef FEAT_QUICKFIX
+    else if (varp == &p_qftf)
+    {
+	if (qf_process_qftf_option() == FALSE)
+	    errmsg = e_invarg;
+    }
+#endif
+
     // Options that are a list of flags.
     else
     {