Problem: GROUP BY on empty ucs2 strings crashed server.
Reason: sometimes mi_unique_hash() is executed with
ptr=null and length=0, which means "empty string".
The branch of code handling UCS2 character set
was not safe against ptr=null and fell into and
endless loop even if length=0 because of poiter
arithmetic overflow.
Fix: adding special check for length=0 to avoid pointer arithmetic
overflow.