patch 8.2.2504: Vim9: crash when using an argument from a closure

Problem:    Vim9: crash when using an argument from a closure.
Solution:   Check if gen_load_outer is NULL. (closes #7821)
diff --git a/src/testdir/test_vim9_func.vim b/src/testdir/test_vim9_func.vim
index cdaba5a..cea25a4 100644
--- a/src/testdir/test_vim9_func.vim
+++ b/src/testdir/test_vim9_func.vim
@@ -1669,6 +1669,18 @@
 
   unlet g:UseArg
   unlet g:UseVararg
+
+  var lines =<< trim END
+      vim9script
+      def Test(Fun: func(number): number): list<number>
+        return map([1, 2, 3], (_, i) => Fun(i))
+      enddef
+      def Inc(nr: number): number
+        return nr + 2
+      enddef
+      assert_equal([3, 4, 5], Test(Inc))
+  END
+  CheckScriptSuccess(lines)
 enddef
 
 def MakeGetAndAppendRefs()