patch 8.2.2848: crash whn calling partial

Problem:    Crash whn calling partial.
Solution:   Check for NULL pointer. (Dominique Pellé, closes #8202)
diff --git a/src/testdir/test_listdict.vim b/src/testdir/test_listdict.vim
index 1b0796d..601dd17 100644
--- a/src/testdir/test_listdict.vim
+++ b/src/testdir/test_listdict.vim
@@ -743,6 +743,7 @@
 
   " should not crash
   call assert_fails('echo reduce([1], test_null_function())', 'E1132:')
+  call assert_fails('echo reduce([1], test_null_partial())', 'E1132:')
 endfunc
 
 " splitting a string to a List using split()