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/ex_cmds2.c

@@ -2832,8 +2832,15 @@ ex_argdelete(exarg_T *eap)
 	if (eap->line2 > ARGCOUNT)
 	    eap->line2 = ARGCOUNT;
 	n = eap->line2 - eap->line1 + 1;
-	if (*eap->arg != NUL || n <= 0)
+	if (*eap->arg != NUL)
+	    /* Can't have both a range and an argument. */
 	    EMSG(_(e_invarg));
+	else if (n <= 0)
+	{
+	    /* Don't give an error for ":%argdel" if the list is empty. */
+	    if (eap->line1 != 1 || eap->line2 != 0)
+		EMSG(_(e_invrange));
+	}
 	else
 	{
 	    for (i = eap->line1; i <= eap->line2; ++i)

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())

src/version.c

@@ -764,6 +764,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    439,
 /**/
     438,
 /**/