MDEV-16119 InnoDB lock->index refers to a freed object after failed ADD INDEX

The problem is hard to repeat, and I failed to create a deterministic
test case. Online index creation creates stubs for to-be-created indexes.
If index creation fails, we could remove these stubs while locks exist
in the indexes. (This would require that the index creation was completed,
and a concurrent DML operation acquired a lock on a record in the
uncommitted index. If a duplicate key error occurs in an uncommitted
index, the error will be reported for the CREATE UNIQUE INDEX, not for
the DML operation that tried to insert the duplicate.)

dict_table_try_drop_aborted(), row_merge_drop_indexes(): If transactional
locks exist on the table, keep the table->indexes intact.

storage/innobase/dict/dict0dict.cc

@@ -507,7 +507,8 @@ dict_table_try_drop_aborted(
 		ut_ad(table->id == table_id);
 	}
 
-	if (table && table->n_ref_count == ref_count && table->drop_aborted) {
+	if (table && table->n_ref_count == ref_count && table->drop_aborted
+	    && !UT_LIST_GET_FIRST(table->locks)) {
 		/* Silence a debug assertion in row_merge_drop_indexes(). */
 		ut_d(table->n_ref_count++);
 		row_merge_drop_indexes(trx, table, TRUE);

storage/innobase/row/row0merge.cc

@@ -2907,7 +2907,8 @@ row_merge_drop_indexes(
 
 	A concurrent purge will be prevented by dict_operation_lock. */
 
-	if (!locked && table->n_ref_count > 1) {
+	if (!locked && (table->n_ref_count > 1
+			|| UT_LIST_GET_FIRST(table->locks))) {
 		/* We will have to drop the indexes later, when the
 		table is guaranteed to be no longer in use.  Mark the
 		indexes as incomplete and corrupted, so that other

storage/xtradb/dict/dict0dict.cc

@@ -507,7 +507,8 @@ dict_table_try_drop_aborted(
 		ut_ad(table->id == table_id);
 	}
 
-	if (table && table->n_ref_count == ref_count && table->drop_aborted) {
+	if (table && table->n_ref_count == ref_count && table->drop_aborted
+	    && !UT_LIST_GET_FIRST(table->locks)) {
 		/* Silence a debug assertion in row_merge_drop_indexes(). */
 		ut_d(table->n_ref_count++);
 		row_merge_drop_indexes(trx, table, TRUE);

storage/xtradb/row/row0merge.cc

@@ -2911,7 +2911,8 @@ row_merge_drop_indexes(
 
 	A concurrent purge will be prevented by dict_operation_lock. */
 
-	if (!locked && table->n_ref_count > 1) {
+	if (!locked && (table->n_ref_count > 1
+			|| UT_LIST_GET_FIRST(table->locks))) {
 		/* We will have to drop the indexes later, when the
 		table is guaranteed to be no longer in use.  Mark the
 		indexes as incomplete and corrupted, so that other