patch 8.1.2273: wrong default when "pos" is changed with popup_atcursor()

Problem:    Wrong default when "pos" is changed with popup_atcursor().
Solution:   Adjust the default line and col when "pos" is not the default
            value. (#5151)
diff --git a/src/ex_cmds.c b/src/ex_cmds.c
index a7f80a8..db5fecd 100644
--- a/src/ex_cmds.c
+++ b/src/ex_cmds.c
@@ -4960,7 +4960,7 @@
 	{
 	    wp = popup_find_preview_window();
 	    if (wp != NULL)
-		popup_set_wantpos_cursor(wp, wp->w_minwidth);
+		popup_set_wantpos_cursor(wp, wp->w_minwidth, NULL);
 	}
 	else if (use_popup != USEPOPUP_NONE)
 	{