MDEV-26458 Crash on ALTER TABLE after DISCARD TABLESPACE

ha_innobase::check_if_supported_inplace_alter(): Do not invoke
innobase_table_is_empty() if the tablespace has been discarded.
That is, native ALTER TABLE in InnoDB will treat an empty table
in the same way as a tablespace whose tablespace has been discarded.
(Note: ALTER TABLE...ALGORITHM=COPY will fail if the tablespace
has been discarded.)

This fixes a crash that was introduced
in commit c7559747 (MDEV-19611).

mysql-test/suite/innodb/r/innodb-alter-timestamp.result

@@ -139,3 +139,15 @@ ALTER TABLE t1 ADD f3 DATE NOT NULL, ALGORITHM=INPLACE;
 ERROR 0A000: ALGORITHM=INPLACE is not supported for this operation. Try ALGORITHM=COPY
 DROP TABLE t1;
 disconnect purge_control;
+#
+# MDEV-26458 SIGSEGV in innobase_table_is_empty() on ALTER TABLE
+#
+CREATE TABLE t(a INT PRIMARY KEY) ENGINE=InnoDB;
+ALTER TABLE t DISCARD TABLESPACE;
+SET sql_mode='NO_ZERO_DATE';
+ALTER TABLE t ADD c DATE NOT NULL;
+Warnings:
+Warning	1814	Tablespace has been discarded for table `t`
+SET sql_mode=DEFAULT;
+DROP TABLE t;
+# End of 10.3 tests

mysql-test/suite/innodb/t/innodb-alter-timestamp.test

@@ -103,3 +103,15 @@ INSERT INTO t1 VALUES (1, now());
 ALTER TABLE t1 ADD f3 DATE NOT NULL, ALGORITHM=INPLACE;
 DROP TABLE t1;
 disconnect purge_control;
+
+--echo #
+--echo # MDEV-26458 SIGSEGV in innobase_table_is_empty() on ALTER TABLE
+--echo #
+CREATE TABLE t(a INT PRIMARY KEY) ENGINE=InnoDB;
+ALTER TABLE t DISCARD TABLESPACE;
+SET sql_mode='NO_ZERO_DATE';
+ALTER TABLE t ADD c DATE NOT NULL;
+SET sql_mode=DEFAULT;
+DROP TABLE t;
+
+--echo # End of 10.3 tests

storage/innobase/handler/handler0alter.cc

@@ -1164,6 +1164,7 @@ ha_innobase::check_if_supported_inplace_alter(
 	/* '0000-00-00' value isn't allowed for datetime datatype
 	for newly added column when table is not empty */
 	if (ha_alter_info->error_if_not_empty
+	    && m_prebuilt->table->space
 	    && !innobase_table_is_empty(m_prebuilt->table)) {
 		DBUG_RETURN(HA_ALTER_INPLACE_NOT_SUPPORTED);
 	}