patch 8.2.3609: internal error when ModeChanged is triggered recursively

Problem:    Internal error when ModeChanged is triggered when v:event is
            already in use.
Solution:   Save and restore v:event if needed.
diff --git a/src/structs.h b/src/structs.h
index 8ffebf3..7bd1ff7 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -4465,3 +4465,8 @@
 
 #define WHERE_INIT {NULL, 0, 0}
 
+// Struct passed to get_v_event() and restore_v_event().
+typedef struct {
+    int		sve_did_save;
+    hashtab_T	sve_hashtab;
+} save_v_event_T;