MDEV-19254 Server crashes in maria_status with partitioned table

Bug was that storage_engine::info() was called with not opened table in
ha_partition::info(). Fixed by ensuring that we are using an opened table.

mysql-test/suite/maria/partition.result

+CREATE TABLE t (a INT, KEY(a)) ENGINE=Aria PARTITION BY KEY(a) PARTITIONS 2;
+SELECT * FROM t PARTITION (p1);
+a
+DROP TABLE t;

mysql-test/suite/maria/partition.test

+--source include/have_partition.inc
+
+#
+# MDEV-19254
+# Server crashes in maria_status / ha_maria::info upon SELECT with partition
+# pruning
+#
+
+CREATE TABLE t (a INT, KEY(a)) ENGINE=Aria PARTITION BY KEY(a) PARTITIONS 2;
+SELECT * FROM t PARTITION (p1);
+DROP TABLE t;

sql/ha_partition.cc

@@ -8304,6 +8304,7 @@ int ha_partition::info(uint flag)
     ulonglong max_records= 0;
     uint32 i= 0;
     uint32 handler_instance= 0;
+    bool handler_instance_set= 0;
 
     file_array= m_file;
     do
@@ -8316,8 +8317,9 @@ int ha_partition::info(uint flag)
             !bitmap_is_set(&(m_part_info->read_partitions),
                            (uint) (file_array - m_file)))
           file->info(HA_STATUS_VARIABLE | no_lock_flag | extra_var_flag);
-        if (file->stats.records > max_records)
+        if (file->stats.records > max_records || !handler_instance_set)
         {
+          handler_instance_set= 1;
           max_records= file->stats.records;
           handler_instance= i;
         }