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/structs.h b/src/structs.h
index 124b059..fd3a1d0 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -2113,7 +2113,8 @@
     POPPOS_TOPLEFT,
     POPPOS_BOTRIGHT,
     POPPOS_TOPRIGHT,
-    POPPOS_CENTER
+    POPPOS_CENTER,
+    POPPOS_NONE
 } poppos_T;
 
 typedef enum {