patch 7.4.1664
Problem:    Crash in :cgetexpr.
Solution:   Check for NULL pointer. (Dominique) Add a test.
diff --git a/src/testdir/test_quickfix.vim b/src/testdir/test_quickfix.vim
index 3908ce0..8da1b6f 100644
--- a/src/testdir/test_quickfix.vim
+++ b/src/testdir/test_quickfix.vim
@@ -692,3 +692,8 @@
   endtry
   quit!
 endfunc
+
+func Test_cgetexpr_works()
+  " this must not crash Vim
+  cgetexpr [$x]
+endfunc