innobase_rename_column_try(): When renaming SYS_FIELDS records
for secondary indexes, try to use both formats of SYS_FIELDS.POS
as keys, in case the PRIMARY KEY includes a column prefix.

Without this fix, an ALTER TABLE that renames a column followed
by a server restart (or LRU eviction of the table definition
from dict_sys) would make the table inaccessible.