SELECT WHERE a= AND b=

Selecting data from memory table with varchar column and hash index over it
returns only first row matched.

Problem was that key length calculation for varchar columns didn't include
number of bytes to store length.

Fixed key length for varchar fields to include number of bytes to store length.


heap/hp_create.c:
  Fixed key length for varchar fields to include number of bytes to store length.
mysql-test/r/heap.result:
  Testcase for BUG#18233.
mysql-test/t/heap.test:
  Testcase for BUG#18233.