patch 8.1.0019: error when defining a Lambda with index of a function result

Problem:    Error when defining a Lambda with index of a function result.
Solution:   When not evaluating an expression and skipping a function call,
            set the return value to VAR_UNKNOWN.

src/testdir/test_lambda.vim

@@ -284,3 +284,9 @@ func Test_named_function_closure()
   call test_garbagecollect_now()
   call assert_equal(14, s:Abar())
 endfunc
+
+func Test_lambda_with_index()
+  let List = {x -> [x]}
+  let Extract = {-> function(List, ['foobar'])()[0]}
+  call assert_equal('foobar', Extract())
+endfunc