patch 8.0.0439: ":%argdel" gives an error for an empty arglist

Problem:    Using ":%argdel" while the argument list is already empty gives an
            error. (Pavol Juhas)
Solution:   Don't give an error. (closes #1546)
diff --git a/src/testdir/test_arglist.vim b/src/testdir/test_arglist.vim
index 0caead8..d18035b 100644
--- a/src/testdir/test_arglist.vim
+++ b/src/testdir/test_arglist.vim
@@ -6,6 +6,10 @@
   call assert_equal(2, argidx())
   %argdelete
   call assert_equal(0, argidx())
+  " doing it again doesn't result in an error
+  %argdelete
+  call assert_equal(0, argidx())
+  call assert_fails('2argdelete', 'E16:')
 
   args a b c
   call assert_equal(0, argidx())