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)

src/testdir/test_arglist.vim

@@ -6,6 +6,10 @@ func Test_argidx()
   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())