INSERT ... SELECT ...

Problem was that when bulk insert is used on an empty
table/partition, it disables the indexes for better
performance, but in this specific case it also tries
to read from that partition using an index, which is
not possible since it has been disabled.

Solution was to allow index reads on disabled indexes
if there are no records.

Also reverted the patch for bug#38005, since that was a workaround
in the partitioning engine instead of a fix in myisam.