patch 9.0.1443: ending Insert mode when accessing a hidden prompt buffer

Problem:    Ending Insert mode when accessing a hidden prompt buffer.
Solution:   Don't stop Insert mode when it was active before. (closes #12237)
diff --git a/src/structs.h b/src/structs.h
index 39248a7..9500139 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -4274,6 +4274,7 @@
     bufref_T	new_curbuf;	    // new curbuf
     char_u	*globaldir;	    // saved value of globaldir
     int		save_VIsual_active; // saved VIsual_active
+    int		save_State;	    // saved State
 } aco_save_T;
 
 /*