at page 1024 with ucs2_bin

Inserting strings with a common prefix into a table with
characterset UCS2 corrupted the table.

An efficient search method was used, which compares end space
with ASCII blank. This doesn't work for character sets like UCS2,
which do not encode blank like ASCII does.

Use the less efficient search method _mi_seq_search()
for charsets with mbminlen > 1.