MDEV-18865 Assertion `t->first->versioned_by_id()' failed in innodb_prepare_commit_versioned

Cause:
* row_start != 0 treated as it exists. Probably, possible row permutations had not been taken in mind.

Solution:
* Checking both row_start and row_end is correct, so versioned() function is used

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

@@ -488,4 +488,13 @@ add column row_end bigint unsigned as row end,
 add period for system_time(row_start,row_end),
 with system versioning;
 set autocommit= 1;
+# MDEV-18865 Assertion `t->first->versioned_by_id()'
+# failed in innodb_prepare_commit_versioned
+create or replace table t (x int) engine=innodb;
+insert into t values (0);
+alter table t add `row_start` bigint unsigned as row start,
+add `row_end` bigint unsigned as row end,
+add period for system_time(`row_start`,`row_end`),
+modify x int after row_start,
+with system versioning;
 create or replace database test;

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

@@ -512,4 +512,16 @@ alter table t
   with system versioning;
 set autocommit= 1;
 
+--echo # MDEV-18865 Assertion `t->first->versioned_by_id()'
+--echo # failed in innodb_prepare_commit_versioned
+
+create or replace table t (x int) engine=innodb;
+insert into t values (0);
+alter table t add `row_start` bigint unsigned as row start,
+              add `row_end` bigint unsigned as row end,
+              add period for system_time(`row_start`,`row_end`),
+              modify x int after row_start,
+              with system versioning;
+
+
 create or replace database test;

storage/innobase/handler/ha_innodb.cc

@@ -10905,6 +10905,9 @@ create_table_info_t::create_table_def()
 
 	heap = mem_heap_create(1000);
 
+	ut_d(bool have_vers_start = false);
+	ut_d(bool have_vers_end = false);
+
 	for (ulint i = 0, j = 0; j < n_cols; i++) {
 		Field*	field = m_form->field[i];
 		ulint vers_row = 0;
@@ -10912,8 +10915,10 @@ create_table_info_t::create_table_def()
 		if (m_form->versioned()) {
 			if (i == m_form->s->row_start_field) {
 				vers_row = DATA_VERS_START;
+				ut_d(have_vers_start = true);
 			} else if (i == m_form->s->row_end_field) {
 				vers_row = DATA_VERS_END;
+				ut_d(have_vers_end = true);
 			} else if (!(field->flags
 				     & VERS_UPDATE_UNVERSIONED_FLAG)) {
 				vers_row = DATA_VERSIONED;
@@ -11034,6 +11039,10 @@ err_col:
 		j++;
 	}
 
+	ut_ad(have_vers_start == have_vers_end);
+	ut_ad(table->versioned() == have_vers_start);
+	ut_ad(!table->versioned() || table->vers_start != table->vers_end);
+
 	if (num_v) {
 		for (ulint i = 0, j = 0; i < n_cols; i++) {
 			dict_v_col_t*	v_col;

storage/innobase/include/dict0mem.h

@@ -1603,10 +1603,14 @@ struct dict_table_t {
 	/** Add the table definition to the data dictionary cache */
 	void add_to_cache();
 
+	/** @return whether the table is versioned.
+	It is assumed that both vers_start and vers_end set to 0
+	iff table is not versioned. In any other case,
+	these fields correspond to actual positions in cols[]. */
 	bool versioned() const { return vers_start || vers_end; }
 	bool versioned_by_id() const
 	{
-		return vers_start && cols[vers_start].mtype == DATA_INT;
+		return versioned() && cols[vers_start].mtype == DATA_INT;
 	}
 
 	void inc_fk_checks()