MDEV-17938: ALTER TABLE error handling accesses freed memory

This regression was introduced in MDEV-11369 instant ADD COLUMN.

prepare_inplace_alter_table_dict(): Avoid dereferencing ctx->new_table
after row_create_table_for_mysql() returns a failure code, because
it will have freed the memory.

storage/innobase/handler/handler0alter.cc

@@ -5018,8 +5018,10 @@ prepare_inplace_alter_table_dict(
 				user_table, ctx->trx);
 
 			if (ctx->need_rebuild()) {
-				ut_ad(!ctx->new_table->cached);
-				dict_mem_table_free(ctx->new_table);
+				if (ctx->new_table) {
+					ut_ad(!ctx->new_table->cached);
+					dict_mem_table_free(ctx->new_table);
+				}
 				ctx->new_table = ctx->old_table;
 			}
 
@@ -5517,7 +5519,7 @@ prepare_inplace_alter_table_dict(
 			break;
 		case DB_TABLESPACE_EXISTS:
 			my_error(ER_TABLESPACE_EXISTS, MYF(0),
-				 ctx->new_table->name.m_name);
+				 altered_table->s->table_name.str);
 			goto new_table_failed;
 		case DB_DUPLICATE_KEY:
 			my_error(HA_ERR_TABLE_EXIST, MYF(0),
@@ -5525,12 +5527,13 @@ prepare_inplace_alter_table_dict(
 			goto new_table_failed;
 		case DB_UNSUPPORTED:
 			my_error(ER_UNSUPPORTED_EXTENSION, MYF(0),
-				 ctx->new_table->name.m_name);
+				 altered_table->s->table_name.str);
 			goto new_table_failed;
 		default:
 			my_error_innodb(error, table_name, flags);
 new_table_failed:
 			DBUG_ASSERT(ctx->trx != ctx->prebuilt->trx);
+			ctx->new_table = NULL;
 			goto new_clustered_failed;
 		}