LEN <= SIZEOF(ULONGLONG)

This bug was caught in the WL#6255 ALTER TABLE...ADD COLUMN in MySQL
5.6, but there is a bug in all InnoDB versions that support
auto-increment columns.

row_search_autoinc_read_column(): When reading the maximum value of
the auto-increment column, and the column only contains NULL values,
return 0. This corresponds to the case when the table is empty in
row_search_max_autoinc().

rb:1415 approved by Sunny Bains