patch 7.4.1123
Problem:    Using ":argadd" when there are no arguments results in the second
            argument to be the current one. (Yegappan Lakshmanan)
Solution:   Correct the w_arg_idx value.
diff --git a/src/ex_cmds2.c b/src/ex_cmds2.c
index 012a1af..59bf945 100644
--- a/src/ex_cmds2.c
+++ b/src/ex_cmds2.c
@@ -2836,6 +2836,7 @@
     int		after;	    /* where to add: 0 = before first one */
 {
     int		i;
+    int		old_argcount = ARGCOUNT;
 
     if (ga_grow(&ALIST(curwin)->al_ga, count) == OK)
     {
@@ -2852,8 +2853,8 @@
 	    ARGLIST[after + i].ae_fnum = buflist_add(files[i], BLN_LISTED);
 	}
 	ALIST(curwin)->al_ga.ga_len += count;
-	if (curwin->w_arg_idx >= after)
-	    ++curwin->w_arg_idx;
+	if (old_argcount > 0 && curwin->w_arg_idx >= after)
+	    curwin->w_arg_idx += count;
 	return after;
     }