MDEV-22660 SIGSEGV on adding system versioning and modifying system column

Second alter subcommand correctly removed VERS_ROW_END flag. We throw
ER_VERS_PERIOD_COLUMNS in such case.

mysql-test/suite/versioning/r/alter.result

@@ -759,3 +759,22 @@ delete from t1;
 set system_versioning_alter_history= keep;
 alter ignore table t1 drop pk;
 drop table t1;
+#
+# MDEV-22660 SIGSEGV on adding system versioning and modifying system column
+#
+create or replace table t1 (a int);
+alter table t1
+add row_start timestamp(6) as row start,
+add row_end timestamp(6) as row end,
+add period for system_time(row_start, row_end),
+with system versioning,
+modify row_end varchar(8);
+ERROR HY000: PERIOD FOR SYSTEM_TIME must use columns `row_start` and `row_end`
+alter table t1
+add row_start timestamp(6) as row start,
+add row_end timestamp(6) as row end,
+add period for system_time(row_start, row_end),
+with system versioning,
+modify row_start varchar(8);
+ERROR HY000: PERIOD FOR SYSTEM_TIME must use columns `row_start` and `row_end`
+drop table t1;

mysql-test/suite/versioning/t/alter.test

@@ -652,3 +652,25 @@ set system_versioning_alter_history= keep;
 alter ignore table t1 drop pk;
 # cleanup
 drop table t1;
+
+
+--echo #
+--echo # MDEV-22660 SIGSEGV on adding system versioning and modifying system column
+--echo #
+create or replace table t1 (a int);
+--error ER_VERS_PERIOD_COLUMNS
+alter table t1
+  add row_start timestamp(6) as row start,
+  add row_end timestamp(6) as row end,
+  add period for system_time(row_start, row_end),
+  with system versioning,
+  modify row_end varchar(8);
+--error ER_VERS_PERIOD_COLUMNS
+alter table t1
+  add row_start timestamp(6) as row start,
+  add row_end timestamp(6) as row end,
+  add period for system_time(row_start, row_end),
+  with system versioning,
+  modify row_start varchar(8);
+# cleanup
+drop table t1;

sql/handler.cc

@@ -7564,6 +7564,12 @@ bool Vers_parse_info::check_sys_fields(const Lex_table_name &table_name,
       row_end= f;
   }
 
+  if (!row_start || !row_end)
+  {
+    my_error(ER_VERS_PERIOD_COLUMNS, MYF(0), as_row.start.str, as_row.end.str);
+    return true;
+  }
+
   if (!can_native ||
       !row_start->is_some_bigint() ||
       !row_end->is_some_bigint())