Added a test case for bug #7520.
table.cc:
  Fixed bug #7520.
  The bug was caused by a wrong calculation of the field max_key_length for
  a TABLE structure when there was an index on a blob field.


sql/table.cc:
  Fixed bug #7520.
  The bug was caused by a wrong calculation of the field max_key_length for
  a TABLE structure when there was an index on a blob field.
mysql-test/t/select.test:
  Added a test case for bug #7520.
mysql-test/r/select.result:
  Added a test case for bug #7520.