This bug in Field_string::cmp resulted in a wrong comparison 
with keys in partial indexes over multi-byte character fields.
Given field a is declared as a varchar(16) collate utf8_unicode_ci
INDEX(a(4)) gives us an example of such an index.
  
Wrong key comparisons could lead to wrong result sets if 
the selected query execution plan used a range scan by 
a partial index over a utf8 character field.
This also caused wrong results in many other cases.