patch 9.0.0260: using freed memory when using 'quickfixtextfunc' recursively

Problem:    Using freed memory when using 'quickfixtextfunc' recursively.
Solution:   Do not allow for recursion.
diff --git a/src/testdir/test_quickfix.vim b/src/testdir/test_quickfix.vim
index 94651af..762fa8d 100644
--- a/src/testdir/test_quickfix.vim
+++ b/src/testdir/test_quickfix.vim
@@ -6351,4 +6351,17 @@
   %bw!
 endfunc
 
+func Test_quickfixtextfunc_recursive()
+  func s:QFTfunc(o)
+    cgete '0'
+  endfunc
+  copen
+  let &quickfixtextfunc = 's:QFTfunc'
+  cex ""
+
+  let &quickfixtextfunc = ''
+  cclose
+endfunc
+
+
 " vim: shiftwidth=2 sts=2 expandtab