existed before the fast-index-creation code was merged, in r1414.

Do not call row_prebuilt_table_obsolete(innodb_table) until after
the table has been renamed to a temporary name.  Otherwise, the
following could happen when creating a clustered index on table T:

(a1) copy T to T#1
(a2) mark T obsolete
(b1) add the name of T to the background drop list
(a3) rename T to T#2 and T#1 to T
(b2) drop the table on the background

These steps are executed by two threads, a and b.

If b1 is executed after a3, all is fine: the name T#2 will be added
to the background drop list.  If b1 is executed before a3, the name
T will be added, and T#2 will survive.