patch 8.2.0657: Vim9: no check if called variable is a FuncRef

Problem:    Vim9: no check if called variable is a FuncRef.
Solution:   Add a type check.
diff --git a/src/testdir/test_vim9_expr.vim b/src/testdir/test_vim9_expr.vim
index 4632272..ace3d2f 100644
--- a/src/testdir/test_vim9_expr.vim
+++ b/src/testdir/test_vim9_expr.vim
@@ -213,6 +213,12 @@
   assert_equal(true, function('g:Test_expr4_equal', [123]) == function('g:Test_expr4_equal', [123]))
   assert_equal(false, function('g:Test_expr4_equal', [123]) == function('g:Test_expr4_is', [123]))
   assert_equal(false, function('g:Test_expr4_equal', [123]) == function('g:Test_expr4_equal', [999]))
+
+  let OneFunc: func
+  let TwoFunc: func
+  OneFunc = function('len')
+  TwoFunc = function('len')
+  assert_equal(true, OneFunc('abc') == TwoFunc('123'))
 enddef
 
 " test != comperator