MDEV-30118 exception in ha_maria::extra

I have not been able to repeat the problem, but the stack trace indicates
that ha_maria::extra() is called with a null file pointer.

This indicates the table has either never been opened or opened and closed,
with file pointer set to NULL, but ha_maria::extra() is still called.

In JOIN::partial_cleanup() we are only checking of table->is_created(),
which will fail if table was created and later closed.

Fixed by clearing table->created if table is dropped.

I added an assert to is_created() to catch the case that the create
flag does not match 'file'.

sql/sql_select.cc

@@ -20320,6 +20320,8 @@ free_tmp_table(THD *thd, TABLE *entry)
     }
     entry->file->ha_drop_table(entry->s->path.str);
     delete entry->file;
+    entry->file= NULL;
+    entry->reset_created();
   }
 
   /* free blobs */

sql/table.h

@@ -1643,7 +1643,11 @@ struct TABLE
   }
 
   /// Return true if table is instantiated, and false otherwise.
-  bool is_created() const { return created; }
+  bool is_created() const
+  {
+    DBUG_ASSERT(!created || file != 0);
+    return created;
+  }
 
   /**
     Set the table as "created", and enable flags in storage engine
@@ -1658,6 +1662,11 @@ struct TABLE
     created= true;
   }
 
+  void reset_created()
+  {
+    created= 0;
+  }
+
   /*
     Returns TRUE if the table is filled at execution phase (and so, the
     optimizer must not do anything that depends on the contents of the table,