cleanup: replace a magic constant with a define

sql/sql_truncate.cc

@@ -319,7 +319,7 @@ bool Sql_cmd_truncate_table::lock_table(THD *thd, TABLE_LIST *table_ref,
     TABLE_SHARE *share= tdc_acquire_share(thd, table_ref, GTS_TABLE | GTS_VIEW);
     if (share == NULL)
       DBUG_RETURN(TRUE);
-    DBUG_ASSERT(share != (TABLE_SHARE*)1);
+    DBUG_ASSERT(share != UNUSABLE_TABLE_SHARE);
 
     versioned= share->versioned;
     sequence= share->table_type == TABLE_TYPE_SEQUENCE;

sql/table.h

@@ -1025,6 +1025,8 @@ struct TABLE_SHARE
   void set_overlapped_keys();
 };
 
+/* not NULL, but cannot be dereferenced */
+#define UNUSABLE_TABLE_SHARE ((TABLE_SHARE*)1)
 
 /**
    Class is used as a BLOB field value storage for

sql/table_cache.cc

@@ -945,7 +945,7 @@ TABLE_SHARE *tdc_acquire_share(THD *thd, TABLE_LIST *tl, uint flags,
       table existed?
       Let's return an invalid pointer here to catch dereferencing attempts.
     */
-    share= (TABLE_SHARE*) 1;
+    share= UNUSABLE_TABLE_SHARE;
   }
   DBUG_RETURN(share);