patch 8.1.0804: crash when setting v:errmsg to empty list

Problem:    Crash when setting v:errmsg to empty list. (Jaon Franklin)
Solution:   Separate getting value and assigning result.
diff --git a/src/testdir/test_eval_stuff.vim b/src/testdir/test_eval_stuff.vim
index c7b970a..f4b3598 100644
--- a/src/testdir/test_eval_stuff.vim
+++ b/src/testdir/test_eval_stuff.vim
@@ -87,3 +87,10 @@
   bwipe!
   call delete('XReadfile')
 endfunc
+
+func Test_let_errmsg()
+  call assert_fails('let v:errmsg = []', 'E730:')
+  let v:errmsg = ''
+  call assert_fails('let v:errmsg = []', 'E730:')
+  let v:errmsg = ''
+endfunc